Name things consistently and idiomatically

[Nadrieril]

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

• "assumed" should be renamed to "built-in";
• Id is a confusing suffix for a rust user: it normally designates integer identifiers only;
• 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. E.g. TraitInstanceRef instead of TraitRef, TraitClause or TraitBound instead of TraitDeclRef, something else instead of TraitClause.
• 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:

• RawStatement -> StatementKind
• RawTerminator -> TerminatorKind
• RawConstantExpr -> ConstantExprKind
• FunIdOrTraitMethodRef -> FnPtrKind
• FunId: merge into FnPtrKind
• TypeId -> make TypeRef that carries GenericArgs, and rename to TypeRefKind
• ItemKind -> ItemContainer
• RawSpan -> SpanData
• Var -> Local
• TraitClause -> WhereTraitClause
• TraitDeclRef -> TraitClause
• TraitRef -> TraitInstanceRef
• AnyTransId -> AnyItemId