KeyInit, KeyExport, KeySizeUser (#1342)
Implements these traits sourced from the `crypto-common` crate. Also implements the `Generate` trait for key generation, replacing the previous `KeyGen` trait, which this commit deprecates in favor of using `KeyInit` and `Generate`. This also adds a `getrandom` feature that forwards through to `crypto-common`, making methods available on the `Generate` trait when enabled.
RustCrypto: Signatures

Support for digital signatures, which provide authentication of data using public-key cryptography.
All algorithms reside in the separate crates and implemented using traits from
the signature crate.
Crates are designed so they do not require the standard library (i.e. no_std)
and can be easily used for bare-metal or lightweight WebAssembly programming.
Crates
NOTE: for RSA signatures see https://github.com/RustCrypto/RSA
Usage
Crates functionality is expressed in terms of traits defined in the signature
crate.
License
All crates licensed under either of
at your option.
Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.