[pallet-revive] last call return data API

[xermicus]

This PR introduces 2 new syscalls: return_data_size and return_data_copy, resembling the semantics of the EVM RETURNDATASIZE and RETURNDATACOPY opcodes.

The ownership of ExecReturnValue (the return data) has moved to the Frame. This allows implementing the new contract API functionality in ext with no additional copies. Returned data is passed via contract memory, memory is (will be) metered, hence the amount of returned data can not be statically known, so we should avoid storing copies of the returned data if we can. By moving the ownership of the exectuables return value into the Frame struct we achieve this.

A zero-copy implementation of those APIs would be technically possible without that internal change by making the callsite in the runtime responsible for moving the returned data into the frame after any call. However, resetting the stored output needs to be handled in ext, since plain transfers will not affect the stored return data (and we don't want to handle this special call case inside the runtime API). This has drawbacks:

• It can not be tested easily in the mock.
• It introduces an inconsistency where resetting the stored output is handled in ext, but the runtime API is responsible to store it back correctly after any calls made. Instead, with ownership of the data in Frame, both can be handled in a single place. Handling both in fn run() is more natural and leaves less room for runtime API bugs.

The returned output is reset each time before running any executable in a nested stack. This change should not incur any overhead to the overall memory usage as only the returned data from the last executed frame will be kept around at any time.

[xermicus]

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