Introduce mutation testing by way of mutagen [0]
- Conditionally add the dev-dependency `mutagen` (using `do_mutate`
flag)
This flag is not very well named but `mutagen` and `mutate` are already
taken?
- Mutate all methods of the `U256` struct that do not require additional
unit tests.
Uses `cfg(all(test, do_mutate), mutate)` - I cannot workout why we need
to check on `test` as well i.e., I don't understand why we cannot use
`cfg(do_mutate, mutate)`?
With this applied test can be run as usual with a stable toolchain. To
run mutagen we use `RUSTFLAGS='--cfg=do_mutate' cargo +nightly mutagen`.
[0] https://github.com/llogiq/mutagen
2e79a0bdc4