Move CTxDestination from script/script to script/standard

This removes CTxDestination (which is really about standard template scripts) from the core script dependency.
Just a random idea: maybe instead of `void SetScriptTo...(...)`,  we could change this API to `CScript GetScriptFor...(...)`. These functions clear out the script and replace it with something else, so unless I'm missing something, there no need to pass in an initial script.

ie, instead of having

    Script script; SetScriptToDestination(script, dest);

everywhere, this would allow

    Script script = GetScriptForDestination(dest);

(I know in the case of SetScriptToDestination there is a return value, but if I saw correctly we ignore it everywhere...)