EVM: deduplicate memory copying code

[Stebalien]

We have some duplicate code in codecopy and calldatacopy (and probably other places:

https://github.com/filecoin-project/builtin-actors/blob/b787886c806f502db487407bf4c0a5fe0931de1e/actors/evm/src/interpreter/instructions/call.rs#L95-L107
https://github.com/filecoin-project/builtin-actors/blob/b787886c806f502db487407bf4c0a5fe0931de1e/actors/evm/src/interpreter/instructions/call.rs#L63-L77

We should try to factor out a helper function and test it. This is critical code that likely has bounds-checking bugs.