Add a RustCrypto based backend for cryptographic COSE operations

[pulsastrix]

Basically the same as was done for openssl in #13, just based on the RustCrypto collection of crates.

The easiest way of using this backend is by enabling the rustcrypto feature.
However, if minimal binary size is desired, you can instead also limit the supported algorithms by adding only the rustcrypto-* features you need.

Resolves #20

[coveralls]

Pull Request Test Coverage Report for Build 10322512694

Details

• 260 of 309 (84.14%) changed or added relevant lines in 15 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 84.95%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/token/cose/crypto_impl/rustcrypto/encrypt/aes_gcm.rs	26	30	86.67%
src/token/cose/crypto_impl/rustcrypto/mac/hmac.rs	34	38	89.47%
src/token/cose/crypto_impl/rustcrypto/key_distribution/aes_key_wrap.rs	24	30	80.0%
src/token/cose/crypto_impl/rustcrypto/sign/ecdsa.rs	45	57	78.95%
src/token/cose/crypto_impl/rustcrypto/mod.rs	15	38	39.47%

Totals
Change from base Build 10239244251:	-0.02%
Covered Lines:	2986
Relevant Lines:	3515

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💛 - Coveralls

[falko17]

Only 5 minor comments, then this should be good to go.

[falko17]

Looks good to me now! Please rebase and then we can merge.