Improve spec of MLOAD, MSTORE, and MSIZE.

[acoglio]

This is in reference to Issue #767, which has all the relevant discussion.

This changes

and

to

and

[acoglio]

@ekpyron Yes, definitely, thanks for catching it.

[acoglio]

@nicksavers Does this look good to you too? If you confirm, you or I can merge it. Thanks.

[nicksavers]

This limit has been a topic of discussion before.

There is EIP-1985: Sane limits for certain EVM parameters and Geth (and some others) have already implemented a different value for MSIZE. You are free to take those values for your interpreter or add your input in the discussion thread.

I suggest not to merge this until the EIP is Final.

[acoglio]

@nicksavers Waiting for the EIP to be final is fine with me.

(We could still fix the overflow problem found by @ekpyron for the time being, even though it's a theoretical case, and then further revise things when the EIP is final. But perhaps we want to avoid too many revisions, especially if the EIP may be finalized soon(?).)

[ekpyron]

IMHO we should merge and fix this independently of that EIP.

[axic]

@nicksavers I think this is independent from EIP-1985. This PR fixes an ambiguity (stack items are 256-bit wide so the offset can be at most 2^256-1), while EIP-1985 introduces stricter limits (which I think should definitely be applied to the YP once the EIP is accepted).