Resolve question_mark clippy lint

    warning: this `match` expression can be replaced with `?`
       --> src/generics.rs:191:20
        |
    191 |           let next = match self.0.next() {
        |  ____________________^
    192 | |             Some(item) => item,
    193 | |             None => return None,
    194 | |         };
        | |_________^ help: try instead: `self.0.next()?`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark
        = note: `-W clippy::question-mark` implied by `-W clippy::all`
        = help: to override `-W clippy::all` add `#[allow(clippy::question_mark)]`

    warning: this `match` expression can be replaced with `?`
       --> src/generics.rs:209:20
        |
    209 |           let next = match self.0.next() {
        |  ____________________^
    210 | |             Some(item) => item,
    211 | |             None => return None,
    212 | |         };
        | |_________^ help: try instead: `self.0.next()?`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark

    warning: this `match` expression can be replaced with `?`
       --> src/generics.rs:227:20
        |
    227 |           let next = match self.0.next() {
        |  ____________________^
    228 | |             Some(item) => item,
    229 | |             None => return None,
    230 | |         };
        | |_________^ help: try instead: `self.0.next()?`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark

    warning: this `match` expression can be replaced with `?`
       --> src/generics.rs:245:20
        |
    245 |           let next = match self.0.next() {
        |  ____________________^
    246 | |             Some(item) => item,
    247 | |             None => return None,
    248 | |         };
        | |_________^ help: try instead: `self.0.next()?`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark

    warning: this `match` expression can be replaced with `?`
       --> src/generics.rs:263:20
        |
    263 |           let next = match self.0.next() {
        |  ____________________^
    264 | |             Some(item) => item,
    265 | |             None => return None,
    266 | |         };
        | |_________^ help: try instead: `self.0.next()?`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark

    warning: this `match` expression can be replaced with `?`
       --> src/generics.rs:281:20
        |
    281 |           let next = match self.0.next() {
        |  ____________________^
    282 | |             Some(item) => item,
    283 | |             None => return None,
    284 | |         };
        | |_________^ help: try instead: `self.0.next()?`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark

src/generics.rs

@@ -188,11 +188,7 @@ impl<'a> Iterator for Lifetimes<'a> {
     type Item = &'a LifetimeParam;
 
     fn next(&mut self) -> Option<Self::Item> {
-        let next = match self.0.next() {
-            Some(item) => item,
-            None => return None,
-        };
-        if let GenericParam::Lifetime(lifetime) = next {
+        if let GenericParam::Lifetime(lifetime) = self.0.next()? {
             Some(lifetime)
         } else {
             self.next()
@@ -206,11 +202,7 @@ impl<'a> Iterator for LifetimesMut<'a> {
     type Item = &'a mut LifetimeParam;
 
     fn next(&mut self) -> Option<Self::Item> {
-        let next = match self.0.next() {
-            Some(item) => item,
-            None => return None,
-        };
-        if let GenericParam::Lifetime(lifetime) = next {
+        if let GenericParam::Lifetime(lifetime) = self.0.next()? {
             Some(lifetime)
         } else {
             self.next()
@@ -224,11 +216,7 @@ impl<'a> Iterator for TypeParams<'a> {
     type Item = &'a TypeParam;
 
     fn next(&mut self) -> Option<Self::Item> {
-        let next = match self.0.next() {
-            Some(item) => item,
-            None => return None,
-        };
-        if let GenericParam::Type(type_param) = next {
+        if let GenericParam::Type(type_param) = self.0.next()? {
             Some(type_param)
         } else {
             self.next()
@@ -242,11 +230,7 @@ impl<'a> Iterator for TypeParamsMut<'a> {
     type Item = &'a mut TypeParam;
 
     fn next(&mut self) -> Option<Self::Item> {
-        let next = match self.0.next() {
-            Some(item) => item,
-            None => return None,
-        };
-        if let GenericParam::Type(type_param) = next {
+        if let GenericParam::Type(type_param) = self.0.next()? {
             Some(type_param)
         } else {
             self.next()
@@ -260,11 +244,7 @@ impl<'a> Iterator for ConstParams<'a> {
     type Item = &'a ConstParam;
 
     fn next(&mut self) -> Option<Self::Item> {
-        let next = match self.0.next() {
-            Some(item) => item,
-            None => return None,
-        };
-        if let GenericParam::Const(const_param) = next {
+        if let GenericParam::Const(const_param) = self.0.next()? {
             Some(const_param)
         } else {
             self.next()
@@ -278,11 +258,7 @@ impl<'a> Iterator for ConstParamsMut<'a> {
     type Item = &'a mut ConstParam;
 
     fn next(&mut self) -> Option<Self::Item> {
-        let next = match self.0.next() {
-            Some(item) => item,
-            None => return None,
-        };
-        if let GenericParam::Const(const_param) = next {
+        if let GenericParam::Const(const_param) = self.0.next()? {
             Some(const_param)
         } else {
             self.next()