Name things consistently and idiomatically

[Nadrieril]

Naming could be improved across the project. Among other things:

• [x] "assumed" should be renamed to "built-in";
• [x] Id is a confusing suffix for a rust user: it normally designates integer identifiers only;
• [x] Similarly, Raw is used in places where a rust user would naturally use a Kind suffix, e.g. TerminatorKind;
• [ ] Traits and clauses and predicates are subtle things, I think better names would help follow what's happening.
• [x] Names don't always match between the rust and ocaml sides. Once we're happy with the rust side we should propagate the changes.

[Nadrieril]

My current list:

• [x] Var -> Local: https://github.com/AeneasVerif/charon/pull/645
• [x] RawStatement -> StatementKind https://github.com/AeneasVerif/charon/pull/805
• [x] RawTerminator -> TerminatorKind https://github.com/AeneasVerif/charon/pull/805
• [x] RawConstantExpr -> ConstantExprKind https://github.com/AeneasVerif/charon/pull/805
• [x] Statement.content -> Statement.kind, Terminator.content -> Terminator.kind, ConstantExpr.value -> ConstantExpr.kind
• [x] FunIdOrTraitMethodRef -> FnPtrKind, FnPtr.func -> FnPtr.kind
• [x] RawSpan -> SpanData, Span.span -> Span.data
• [x] TypeVar/RegionVar/ConstGenericVar -> TypeParam/RegionParam/ConstGenericParam
• [x] TraitClause -> TraitParam
• [x] AnyTransId -> ItemId, AnyTransItem -> ItemRef, AnyTransItemMut -> ItemRefMut
• [ ] DeBruijnVar -> Var? (this one is likely to cause clashes in aeneas)
• [x] ItemKind -> ItemSource, FunDecl.kind -> FunDecl.src, GlobalDecl.kind -> GlobalDecl.src
• [x] TraitDecl.parent_clauses -> TraitDecl.implied_clauses, TraitImpl.parent_trait_refs -> TraitImpl.implied_trait_refs
• [ ] TraitRef -> TraitExpr?
• [ ] FnPtr -> FnRef?