srml-contracts: Contract calls/instantiations to return exit statuses (#3320)

* srml-contracts: Rename ext_scratch_copy to ext_scratch_read.

This is to disambiguate from the next ext_scratch_write function.

* Remove unnecessary OutputBuf and EmptyOutputBuf.

* Replace VmExecError with a result type of custom structs.

* Do not drop the scratch buffer on traps and regular returns.

This just reduces the number of allocations required during nested
contract calls and instantiations.

* Semantics for returning a status code and data from contract calls.

* Remove CallReceipt and InstantiateReceipt.

With forthcoming changes to return data from instantiate calls, the two
types of receipts become very similar to each other and to
ExecReturnValue. Instead, replace them with ExecReturnValue and
a regular 2-tuple in the case of instantiation.

* Modify contract function signatures to allow returning status codes.

* Introduce ext_sandbox_write runtime function.

* Test all the things....