Most importantly, intern all the strings we put into the AST* nodes.
(the AST_Module* owns them)

This should save us some memory, but it also improves performance pretty
substantially since now we can do string comparisons very cheaply.  Performance
of the interpreter tier is up by something like 30%, and JIT-compilation times
are down as well (though not by as much as I was hoping).

The overall effect on perf is more muted since we tier out of the interpreter
pretty quickly; to see more benefit, we'll have to retune the OSR/reopt thresholds.

For better or worse (mostly better IMO), the interned-ness is encoded in the type
system, and things will not automatically convert between an InternedString and
a std::string.  It means that this diff is quite large, but it also makes it a lot
more clear where we are making our string copies or have other room for optimization.