Add precompiles to support signature checking on provided and historic headers

[nategraf]

Description

This PR modifies the getValidatorAddress and numberValidators precompiles accept a block number to query a historic validator set, and adds the

Tested

Unit tests

Other changes

• Change an instance of the name "infrastructure fund" to "community fund"
• Change the unused VerifySeal function on the Istanbul consensus engine to have a more useful verification of the seal. (The previous definition was an incomplete implementation of verification and was almost certainly implemented to satisfy the consensus.Engine interface.)
• Modified a few comments to describe more accurately the implementation.
• Move core/evm.go to core/vm/context.go to allow code sharing with contract_comm/evm.go and testing in core/vm/contracts_test.go
• Add helper function to calculate celo precompile addresses

Related issues

• Related Precompile to detect downtime for slashing #695
• Related Precompile to detect double signing for slashing #696

Backwards compatibility

Not backwards compatible due to a change in the validator set precompiles.

[asaj]

Please update the "Other changes" section of the PR

[asaj]

I'm worried that returning a different error will mean we won't log the more informative error

[nategraf]

It's my understanding that errors returned from the EVM need to be deterministic so I wanted to avoid relaying error messages that may change in future updates.

[asaj]

Interesting, why do they need to be deterministic?

[nategraf]

I had just assumed error messages to be included under the umbrella of EVM determinism, but maybe they aren't. If not, I'm happy to put the rlp errors back.