RETURNDATASIZE / RETURNDATACOPY don't work for delegate calls

[charles-liu]

There are two ways to get return data of a delegate call:

• For static-size types, caller passes a memory offset and the length of return data through the stack, then read the memory from the offset.
• For dynamic-size types, caller execute RETURNDATACOPY to get the size of return data and execute RETURNDATACOPY to copy return data to the memory.

Consider both the above second scenario and #553 , RETURNDATASIZE / RETURNDATACOPY should follow the rules:

• If a delegate call has been executed in this receiving session, get the return data from the context.
• Otherwise get it from the calldata of the callback transaction.