Currently when debug printing the `RecoverableSignature` we do so byte
by byte, this means that the output differs depending on the endianess
of the machine. If instead we serialize the signature in compact form
then the output is the same irrespective of the endianess.
With this applied the following two commands now pass:
```
cargo test test_debug_output --features=recovery
```
cross test --target powerpc-unknown-linux-gnu test_debug_output --features=recovery
```
Fixes: #375
It's super dangerous to use Cargo features for this, since they can be set
accidentally (or maliciously by any crate in a user's entire dep tree). Instead
we can just require users set `RUSTFLAGS` appropriately, which we can easily
do in our fuzzing scripts.
This feature was not useful for Cargo users, since Cargo does not give you
the kind of fine-grained control over C library linkage that you need. So
it was just unnecessarily confusing and would cause the build to break if
you enabled it accidentally, say, with --all-features.