Add basic support for array lengths in types

This recognizes `let a = [1u8, 2, 3]` as having type `[u8; 3]` instead
of the previous `[u8; _]`. Byte strings and `[0u8; 2]` kinds of range
array declarations are unsupported as before.

I don't know why a bunch of our rustc tests had single quotes inside
strings un-escaped by `UPDATE_EXPECT=1 cargo t`, but I don't think it's
bad? Maybe something in a nightly?

crates/hir_ty/src/lower.rs

@@ -174,6 +174,8 @@ impl<'a> TyLoweringContext<'a> {
             }
             TypeRef::Array(inner) => {
                 let inner_ty = self.lower_ty(inner);
+                // FIXME: we don't have length info here because we don't store an expression for
+                // the length
                 TyKind::Array(inner_ty, dummy_usize_const()).intern(&Interner)
             }
             TypeRef::Slice(inner) => {