Adds an initial implementation of DSA (see #8)
The following things work when tested against OpenSSL:
- The generated keys are valid and can be imported and exported from/to their DER/PEM representation
- Signatures generated by this library can be successfully verified
- Signatures can be imported and exported from/into their DER representation
- Signatures generated by OpenSSL can be successfully imported and verified
Bumps the following crate dependencies, which all now depend on
`generic-array` v0.14:
- `elliptic-curve` v0.4.0
- `k256` v0.3.0
- `p256` v0.3.0
- `p384` v0.2.0
Also re-exports `generic-array` from the `elliptic-curve` crate rather
than depending on it directly, which simplifies upgrades.
The only thing the `signature` crate uses `alloc` for is the
`Signature::to_vec` method, which is trivially accomplished otherwise as
`sig.as_ref().to_vec()` or various other ways.
By getting rid of it, we can completely get rid of the `alloc` feature,
and with that reduce the MSRV back to 1.31.
It might be worth considering a 1.0 release with a higher MSRV so we can
leverage `TryFrom`, but for now, this provides wider compatibility by
removing a single (mis)feature.
Tony Arcieri