don't allocate a new arena in h_bind, use the existing one

Rationale: If memory allocation fails in the inner parse and we
longjump up the stack, the temporary arena will be missed and leak.

NB: This change means that any allocations done by the continuation
(in the form of new parsers, probably) will persist for the
lifetime of the parse result. Beware of wasting too much memory
this way! The bind continuation should generally keep dynamic
allocations to a minimum.