Remove core dependencies from CScript

As an ongoing effort to break out script verification. I hope this doesn't conflict with @sipa's or @jtimon's next steps.

This work moves the logic for writing custom core classes to scripts out of CScript. Those classes can describe how their data should be written to a script, and CScript does so without further knowledge.

Please ignore the entire PR's diffstat while reviewing as it won't be at all helpful. Individual commits will make much more sense.

Also, CScriptBinaryData is a pretty terrible name but I couldn't come up with anything better. Suggestions welcome.
Hmm, this is not strict enough; the size needs to be at most 75 for this to work.

An alternative is not serializing the size in here, and doing that inside CScript::operator<<, which selects based on the size.