Rustdoc-Json: More inconsistant names

[aDotInTheVoid]

• [x] use's are refered to as Import's (#128667)
• [x] FnDecl uses short Fn, but Function and FunctionPointer don't abbreviate (#128667)
• [x] OpaqueTy uses short Ty, but full type uses everywhere else (removed in #127276)
• [x] Header not called FunctionHeader (#128667)
• [x] Header has fields async_, const_ and unsafe_, but should be is_async (etc), like rest of types. (#128667)
• [x] ItemKind has Primitive, but ItemEnum has PrimitiveType (#102321)
• [x] ItemEnum::ForeignType should be ExternType, as foreign refers to a different rust crate, but it's actually a non rust type (#128667)
• [ ] AssocItemConstraint's `binding field should probably be renamed as followup to the binding->contraint change in #128667
• [ ] StructField isn't just used for struct's, but also variant fields
• [x] WherePredicate::RegionPredicate shoud probaby use lifetime instead. The region/lifetime thing has a history on the compiller side (https://github.com/rust-lang/compiler-team/issues/634, https://github.com/rust-lang/rust/issues/110254), but we should avoid Region on the user-facing side. Done in #127289

Somewhat of a followup to #94889

@rustbot modify labels: +T-rustdoc +A-rustdoc-json

[its-the-shrimp]

• [x] the default field of the ItemEnum::AssocType variant has a misleading name since associated types in impl blocks are also represented with this variant, and in their case that's not the "default" type (#128667)