Use hash tables even for context-sensitive macros

Use hash tables even for context-sensitive macros

Normally, contexts aren't used with a large number of macros, but in
case someone does, do use hash tables for those as well.  This
simplifies the code somewhat, since *all* handling of macros is now
done via hash tables.

Future note: consider if it wouldn't be better to allow struct
hash_table to be allocated by the caller, instead of being allocated
by the hash table routine.