Don't include destructors in samples

[Chris Warburton]

The choose_sample command currently invokes (eventually, nested some
layers down) the lowercase-names function. This extracts all function
names *and* destructor names, even though the latter will never
contribute to any theorem, since we replace references to constructors
and destructors with their 'constructor-' and 'destructor-' wrappers.

We *might* be able to just rip the destructors stuff out of
lowercase-names, but we should check where else it's used.