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/test.rs

@@ -564,7 +564,7 @@ fn mk_tests(cx: &TestCtxt) -> P<ast::Item> {
     let static_lt = ecx.lifetime(sp, keywords::StaticLifetime.name());
     // &'static [self::test::TestDescAndFn]
     let static_type = ecx.ty_rptr(sp,
-                                  ecx.ty(sp, ast::TyKind::Vec(struct_type)),
+                                  ecx.ty(sp, ast::TyKind::Slice(struct_type)),
                                   Some(static_lt),
                                   ast::Mutability::Immutable);
     // static TESTS: $static_type = &[...];