Fix memory leaks

Fix memory leaks

Use the existing, but unused, term_routine member of struct recode_step to
register finalisers.

Also add step_table_term_routine, as adding separate finalisers for
different table types is nicer than having to add finalisers for a huge
number of transformers.

Zero-initialise all allocated memory, so we can assume it, and as a
defensive measure.

Move struct ucs2_to_byte from recodext.h into recode.c, its only user.