stringtable: add diagnostic logging mode

stringtable: add diagnostic logging mode

With INTERNED_STRING_INSTRUMENT set, a /tmp/hash.log file is written
out to record each string being interned.  Looking through that is a
good way of tuning the hash function, as well as what
staptree/etc. variables are reverted to std::string form.  (The hint
is that if strings are hardly ever interned in the log, then maybe
they are not reused either.  But that could instead be because they're
being reused exclusively in interned string_ref form, so don't need to
be rehashed.)