hl_mod: don't memoize highlight::codeGenerator objects

hl_mod: don't memoize highlight::codeGenerator objects

Making them immortal doesn't seem worth it, since doing immortal
allocations after process startup leads to fragmentation.  While
the allocations made by highlight are small, those small
allocations can break up contiguous regions and prevent
consolidation by the malloc implementation.

Since instantiating code generators doesn't seem too expensive,
just use and delete them ASAP.