Add match guard to AST

[cmester0]

I have added the match guard to the AST. Currently no backend supports these, so this makes the rejection of this feature explicit. Furthermore we can now add a phase for removing match conditionals. This should also allow us to rewrite record pattern matching into guarded match statements (should help with #180 / #168).

[cmester0]

We can add a phase that unrolls (guarded) matches, e.g.

match foo {
    Foo { a, .. } if a > 10 => 1,
    Foo { b, .. } if b => 2,
    _ => 3,
};

becomes

if let Foo { a, .. } = foo && a > 10 {
  1
} else if let Foo { b, .. } = foo && b {
  2
} else if let _ = foo {
  3
} else {
  panic!()
};

Which should allow us to do matching on records by moving the projection to the right side

if let a = foo.a && a > 10 {
  1
} else if let b = foo.b && b {
  2
} else if let _ = foo {
  3
} else {
  panic!()
};

Which can be refolded into match statements, as support for let_chains (experimental feature) e.g. if let _ && _ also seems to be missing.

[W95Psp]

Hi Lasse, thanks for the PR, supporting guards on arms would indeed be great.

Both let guards on arms and let chains on if and arms are nightly: I wonder if we want to support any of those, maybe we should only be supporting .. if <bool-expr> => ...

But as you say, having (chainable) let guars on arms might help us fixing #168: that might be a nice intermediate representation. We could rewrite matches that includes patterns some backends don't support into matches with guards, and then have a phase that "multiplexes" such guarded matches into nested matches.

Otherwise the PR looks good, there's just a few TODO to fix. If you want i can fix the ones on the visitors, tell me. (I plan to write a PPX for generating those at some point, this code is not nice to update)

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