`i256` and `u256`
- operators now use the same overflow convention as primitives
- implement `<<` and `-` (previously just `>>` and `+`)
- implement `Ord` correctly (the previous `PartialOrd` was broken)
- correct `i256::SIGNED` to `true`
The `Int`-trait is extended with `trailing_zeros`, `carrying_add`, and
`borrowing_sub`.
Distribute everything from `libm/` to better locations in the repo.
`libm/libm/*` has not moved yet to avoid Git seeing the move as an edit
to `Cargo.toml`.
Files that remain to be merged somehow are in `etc/libm`.