This extracts the SHA256d code from being split across two modules and puts it
in one module, under serialization.
The code is unchanged except for three deleted tests:
* `sha256d_flush` in `sha256d_writer` (not a meaningful test);
* `transactionhash_debug` (constructs an invalid transaction hash, and the
behavior is tested in the next test);
* `decode_state_debug` (we do not need to test the Debug output of
DecodeState);
b36fe8f937