generalize the builtin struct type to adt to match rustc

[nikomatsakis]

Chalk currently has a notion of struct types but rustc has the more general adt. Also, chalk expects a DefId but in rustc we use an AdtDef (a pointer to an interned structure).

We should align with rustc here:

• Rename TypeName::Struct to TypeName::Adt
• Rename StructId<I> to AdtId<I>
• Extend Interner trait to have I::InternedAdtId and adapt AdtId<I> to be a wrapper around InternedAdtId (instead of I::DefId, as today)
• We can map InternedAdtId to the same type as DefId in our integration, rustc can use &'tcx AdtDef
• Rename StructDatum and StructDatumBound to use the Adt terminology -- we don't actually have to change the list of fields for now, though eventually we may want to do so to allow rustc to more cheaply give access from the AdtDef

This issue has been assigned to @Mcat12 via this comment.

[AzureMarker]

This seems straightforward enough, I'll give it a shot.

@rustbot claim

[rustbot]

Error: Parsing assign command in comment failed: ...'bot assign' | error: specify user to assign to at >| ''...

Please let @rust-lang/release know if you're having trouble with this bot.