libsyntax: clearer names for some AST parts

libsyntax: clearer names for some AST parts

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`.