Find and fix RGBASM memory leaks

[Rangi42]

• [ ] PURGE or REDEF of an EQUS symbol does not free its previous string contents.
• [ ] Use Bison to free memory when a token is deleted
• [ ] Files are never unmaped if a single macro or REPT block occurred within
• To be continued

[ISSOtm]

Believe it or not, but you can't free an EQUS :) I tried, and it explodes with a bunch of funny colors. The main problem is an expansion deleting itself; the solution is to ref-count strings. (This overlaps with #650.)

A large chunk of the problem is the lack of cleanup when done with assembling.

[ISSOtm]

Note that Bison's token destructors might help with this a bit.

[Rangi42]

ASAN is enabled, but disables checking for memory leaks for now with detect_leaks=0.

[Rangi42]

See about using an 'std::shared_ptr aliasing constructor' for dealing with string_views that preserve the lifetime of a backing std::string (or mmap or whatever).