Chainspecs and cross-client,cross-chain tests

[meowsbits]

:warning: Based on #65 to greatly simplify diff. Please review that first.

:rabbit: Inclusively supersedes #61.

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TL;DR

• Can read and write Parity JSON chainspecs
  • https://github.com/etclabscore/multi-geth/tree/feat/x-chainspec-types-and-tests/params/parity.json.d

> make test-multigeth-chainspecs

• Can generate State and Difficulty tests for any supported chain config/fork
  • https://github.com/meowsbits/tests/commit/fd9819cf7edd94b65aaf742e281378b9b42f020f
  • https://github.com/meowsbits/tests/commit/7504868d88aedb877f6a6f43e987aea1ec00e86d

> make tests-generate

• Can prove that multi-geth's Features-not-Forks implementation is test-equivalent to ethereum/go-ethereum's Foundation-only implementation

> make test-multigeth-features

This is, to a large extent, a revisitation and rebuild of https://github.com/multi-geth/multi-geth/pull/103, which was built adhoc in preparation of the ETC Atlantis fork. With ETC Agharta around the corner, the need for cross-client compatible non-Foundation chain tests has risen again.

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Full Notes

:bookmark: Please see https://github.com/etclabscore/multi-geth/wiki/PR:-Chainspecs-and-cross-chain,cross-chain-test-generation.

[meowsbits]

Going to merge.

Since this is so big and architecturally significant, and touches several chain configs and consensus implementations, I think we should put the resulting master version thru sanity and manual "regression" tests to make sure we don't see any obvious unexpected or missing behavior.