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`.

src/libsyntax/diagnostics/plugin.rs

@@ -215,7 +215,7 @@ pub fn expand_build_diagnostic_array<'cx>(ecx: &'cx mut ExtCtxt,
 
     let ty = ecx.ty(
         span,
-        ast::TyKind::FixedLengthVec(
+        ast::TyKind::Array(
             ecx.ty(
                 span,
                 ast::TyKind::Tup(vec![ty_str.clone(), ty_str])