More type metadata

[jberthold]

Emit metadata for all types that matter, in particular reference types and array types.
Function types are only referred to as strings (elided in the golden tests), some other types are still not considered but they don't seem relevant to our purpose.

Also included:

• add type of TyConst in RigidTy::Array case to collected types in collect_ty
• optionally keep json files after running test-ui

RigidTy	Metadata	Information
Bool	PrimitiveType	RigidTy	existed
Char	PrimitiveType	RigidTy	existed
Int(IntTy)	PrimitiveType	RigidTy	existed
Uint(UintTy)	PrimitiveType	RigidTy	existed
Float(FloatTy)	PrimitiveType	RigidTy	existed
Adt(AdtDef, GenericArgs)	EnumType		existed
	StructType		existed
	UnionType		ADDED
Foreign(ForeignDef)
Str	PrimitiveType	RigidTy	ADDED
Array(Ty, TyConst)	ArrayType	ElemType, len	ADDED
Pat(Ty, Pattern)
Slice(Ty)	ArrayType	ElemType	ADDED
RawPtr(Ty, Mutability)	PtrType	PointeeType	ADDED
Ref(Region, Ty, Mutability)	RefType	PointeeType	ADDED
FnDef(FnDef, GenericArgs)	FunType	type string only	ADDED
FnPtr(PolyFnSig)	FunType	type string only	ADDED
Closure(ClosureDef, GenericArgs)	FunType	type string only	ADDED
Coroutine(CoroutineDef, GenericArgs, Movability)
Dynamic(Vec<Binder>, Region, DynKind)
Never			?
Tuple(Vec)	TupleType	Component types	ADDED
CoroutineWitness(..)

[gtrepta]

LGTM. I can get my TyCollector changes based off of this.