Add interface for getting preprocessed term (#5798)

Add interface for getting preprocessed term (#5798)

Several places, e.g. in quantifiers, requiring knowing what the theory-preprocessed form of a node is.

This is required for an improvement to our E-matching algorithm, which requires knowing what the preprocessed form of ground subterms of triggers are.

Note that I'm not 100% happy with adding a new interface to Valuation, but at the moment I don't see a better way of doing this. On the positive side, this interface will make a few other things (e.g. the return value of OutputChannel::lemma) obsolete.