`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`.
Currently we run logspace tests for extensive tests, but there isn't any
reason we couldn't also run more kinds of tests more extensively (e.g.
more edge cases, combine edge cases with logspace for multi-input
functions, etc). As a first step toward making this possible, make
`extensive` a new field in `CheckCtx`, and rename `QuickSpaced` to
`Spaced`.
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`.