Implement keccak256 operator and softfork

[Rigidity]

Implements CHIP-36, using the implementation originally provided by @arvidn in #403 for CHIP-34.

New operators

There is 1 new operator enabled under softfork 1:

operator	opcode
keccak256	62

Cost

operator	base cost	per argument	per byte
keccak256	50	160	2

As always, there's also a cost of 10 per byte that's allocated for the return value.

Activation

Since this is a softfork, the extension will only be available when the ENABLE_KECCAK flag is set, which it will be after the softfork activates.

The operators will only be available for programs (<prg>) executing under (softfork <cost> 1 <prg> <arg>)

Hard fork

There's an option to hard-fork these operators out of the softfork-guard by specifying ENABLE_KECCAK_OPS_OUTSIDE_GUARD.

Tests

The test vectors are generated using the python wheel from eth_hash.auto import keccak.

[coveralls-official]

Pull Request Test Coverage Report for Build 11730142584

Details

• 29 of 29 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 93.406%

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Totals
Change from base Build 11574796277:	0.06%
Covered Lines:	5595
Relevant Lines:	5990

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💛 - Coveralls

[arvidn]

did you generate the fuzzing corpus and run the fuzzer for a while?