Add more valid ECDSA test vectors

[marsella]

There are a bunch of test vectors for ECDSA in RFC 6979. Some of these are for invalid curve-hash combinations. Decide which of these we'd like to include in the repo, then instantiate the needed versions of ECDSA and implement the test vectors.

This might be blocked on #98, in order to be able to switch out different hash functions easily. The current version fixes it to SHA-256, but that only works with two of our curves (P-224 and P-256).

• Decide which hash-curve combos to instantiate
• Add all the 6979 test vectors
• Do a time-limited search for other test vector sources

[marsella]

Here's a permalink to the ECDSA sign tests from the previous version of the implementation.

[marsella]

#32 also references some ECDSA test vectors from NIST that are already in Cryptol syntax. Add these too, if appropriate!