Syntactic analysis: Outstanding tasks

[fmease]

• bare trait object types
• fn delegation
• reuse impl (brand new, fn delegation)
• use closures & use blocks (async use {}, gen use {}, async gen use {})
• contract annotations
• builtin # offset_of(…)
• guard patterns
• reject num lit suffixes for tuple field accesses
• split float lits to parse consequent tuple field accesses (and hope that we'll find a nicer approach in the future)
  • to be used for field access exprs & inside builtin syntax offset_of
• support generic args in delegation path trees
• reject inner attrs in certain block exprs
• const block items