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.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Signed-off-by: xermicus <cyrill@parity.io>
Co-authored-by: command-bot <>
Co-authored-by: PG Herveou <pgherveou@gmail.com>