Simple patch series to fix up all the warnings a rustc compile is giving at the moment. It also fixes a NOTE in `to_bytes.rs` to remove the `to_bytes::iter_bytes_<N>` functions.

r? @catamorphism 

This could probably refactored to more closely mirror the code for `@objects`, but I'm not inclined to do so.

This passes make check now.

This only changes the directory names; it does not change the "real"
metadata names.

This improves -O0 compile times dramatically.

This can perhaps be restored in the future. For now this is a precursor to
making typedefs work as expected.

Implement SmallIntSet (refer #4984)

Fix issue #6209, and some related issues about constant expression
 - unmatched type between arms
 - unmatched type in range

The error message is extremely unideal.