Tracking issue for unsupported rust features

[Nadrieril]

This issue tracks language features we don't support yet.

• dyn Trait: Add support for dyn Trait #123
• for<'a> bounds: Add support for quantified where clauses (for <'a>) #67
• String constants: Add support for string literals #72
• enum as isize casting: Support enum-to-discriminant casting #91
• Retain type aliases: Export type aliases in LLBC #134
• Associated type equalities: Hide associated types equality constraints #127
• Mutually recursive traits: Add support for mutually recursive trait definitions #94
• Emit provided trait methods differently: Impossible to override provided trait methods aeneas#70
• Correct Drop semantics: Retrieve the result of drop elaboration #152
• vec![...] macro: Add support for vec macro #165
• Unsafe code: Support unsafe code #280
• Generalized associated types (GATs): Support for Generalized Associated Types #477
• Return-position impl trait in trait (RPITIT): Support return position impl trait in trait #478
• pre-/post-conditions and invariants: Add support for spec annotations #64