evm2wasm specific EEI methods

[axic]

This topic has been touched upon a few times, but never discussed in actual depth.

Not after doing some protoyping with runevm it seems the following features are missing:

• getTxGasLimit
• getCallDepth (EIP3 proposes this)
• getCallType / inStaticMode

Obviously all this is needed if gas calculation is done by evm2wasm as opposed to be part of the EEI.

[s1na]

Another method required for calculating gas as part of runevm is isAccountEmpty(address), which is used in CALL (and for future constantinople support in EXTCODEHASH). Is there any objections to adding this method? otherwise I'll make a PR.