std.builtin.Type: rename Pointer.Size fields to lowercase

This matches established naming conventions. Now is an opportune time to
make this change, since we're already performing breaking changes to
`std.builtin.Type`.

lib/std/builtin.zig

@@ -613,10 +613,10 @@ pub const Type = union(enum) {
         /// This data structure is used by the Zig language code generation and
         /// therefore must be kept in sync with the compiler implementation.
         pub const Size = enum(u2) {
-            One,
-            Many,
-            Slice,
-            C,
+            one,
+            many,
+            slice,
+            c,
         };
     };