Support for local imperative features

[romac]

Add support for local mutation (ie. no mutable references, just mutable variables/parameters within a function).

• Blocks - { a; b; c } (Block)
• Declare variables - let mut x: i32 = 0; body (LetVar)
• Assignments - vd = value (Assignment)
• Field assignment - expr.field = value (FieldAssigment)
• Return statement - return expr (Return) => tracked here Extract early returns in functions #71
• Old - old(expr) (Old)
• Snapshot - snapshot(expr) (Snapshot)

Start thinking about:

• Mutable maps
• Arrays

[yannbolliger]

The big question here, is what we want to do with references. For now, we allowed references because we knew that mutability was prohibited. As soon as mutability comes back, we need to revise, how we deal with references.

EDIT: question answered here in #98.