…pe of other AstBuilder.stmt* functions

I am using `ThinAttributes` rather than a vector for attributes
attached to generics, since I expect almost all lifetime and types
parameters to not carry any attributes.

…s list.

…ached binding in generics.

These weren't flagged by the lint because they were still technically
created somewhere... if you created them before, somehow.

This applies the HIR changes from the previous commits to the AST, and
is thus a syntax-[breaking-change]

Renames `PatKind::Vec` to `PatKind::Slice`, since these are called slice
patterns, not vec patterns. Renames `TyKind::Vec`, which represents the
type `[T]`, to `TyKind::Slice`. Renames `TyKind::FixedLengthVec` to
`TyKind::Array`.

…, r=Manishearth

 This commit makes the return type of AstBuilder.stmt_let_typed match the return type of other AstBuilder.stmt* functions. This avoids unnecessary boxing/unboxing whenever Stmt's are stored in a Vec, which is the default use case.nnThis is a potentially plugin breaking change.

…orite-data-structure, r=pnkfelix

 Contains a syntax-[breaking-change] as a separate commit (cc rust-lang#31645).nnAlso renames slice patterns from `PatKind::Vec` to `PatKind::Slice`.

…r=eddyb

 First step for rust-lang#34761