Substantial surface language redesign

[fmease]

• [x] complete the trait lowering step, specifically the synthesis of the accessors
• [ ] represent record literals in Lo-AST & HIR and lower & resolve record literals
• [x] lower trait literals to record literals (& add FIXME for rejecting attempts to construct records through trait literals)
• [x] lower given-declarations to function decls + @context (not part of the surface lang) + record literal
• [x] get rid of parse_associated_declaration, just parse_declaration inside of data decls & reject non-functions in the lowerer (use-decls: todo, rest: invalid)
• [ ] write a bunch of new UI tests and finish updating the older ones
• [x] support the pattern let _ (i.e. LocalBinder::Discarded) in the lowerer & the resolver (currently: crash in the lowerer)
• [x] fix attribute validation which got trashed in this patch
  • in some shape or form we want to support the newly added syntactic constructs (wildcards, record & trait decls, etc.)
  • the question is how fine-grained, leading to the next point
  • bitfields are not a great solution for attribute targets and it became more and more obvious
  • they are not versatile, flexible or extensible enough cmp. to normal Rust functions
  • we always wanted to disallow @public on the root module but couldn't (in this system which I tried to largely stick to)
  • special care needs to be taken for associated decls (constructors, record & trait fields), esp. wrt. @public
  • the current system is awkward
• [x] define @given and reject it in the lowerer as unimplemented
• [ ] (not sure if blocking, otherwise as FIXME) lower attributes on record & trait fields to … parameter attributes
• [ ] fix the most offensive @Temporarys & @Tasks intro'd in this PR

[fmease]

Non-blocking TODOs (non-exhaustive):

• start writing up some sort of reference / specification since the semantics are a tad more complicated & it starts getting harder to keep everything in my head esp. when stuff changes so drastically so quickly
• perform similar refactorings to the rest of the code base
  • inline error module for -> Diagnostic functions
  • (maybe) split into smaller fns (only where sensible ofc)
• proper documenter support for record & trait decls
  • reconstruct them to their pre-lowered form
  • render docs & other attrs on record & trait fields
• documenter support for param docs (which are a natural consequence of supporting field attrs via param attrs)
• deny shadowing for ast::Parameters & add tests

[fmease]

Going to postpone the unticked tasks from the description.