[WIP] add ed25519 precompile (EIP665)

[oberstet]

See also:

• https://github.com/ethereum/EIPs/blob/master/EIPS/eip-665.md
• [WIP] add ed25519 sig verification precompile go-ethereum#16453

[oberstet]

I am stuck on getting unit tests working though (https://github.com/ethereum/pyethereum/pull/862/files#diff-a7fd7f958e91cefb06e0febd2d19dd05R1240): https://gist.github.com/oberstet/07f76327d5e339a0757be12e9b794ad3

What am I doing wrong? =(

[oberstet]

The PR can use 2 Ed25519 implementations:

• PyNaCl (if available: this is a cffi wrapped NaCl native extension running on both CPy and PyPy)
• an embedded pure Python implementation

[oberstet]

CI is green .. oh, I see;)

  # XXX: For now we're only performing minimal CI checks as most tests are
  # broken. Tests will be individually added here as they're fixed.
- make lint-minimal
 - make test-passing

and https://github.com/ethereum/pyethereum/blob/develop/ethereum/tests/test_contracts.py isn't in this list https://github.com/ethereum/pyethereum/blob/develop/Makefile#L49

[oberstet]

FIXME: this import is wrong here