Cleanup, no functional change:

Cleanup, no functional change:
In libroff.h, nothing was left except the eqn(7) parser interface, which
isn't really part of the roff(7) parser, so rename it to eqn_parse.h.

While here, move struct eqn_def to eqn.c because that's the only
file using it, and let eqn_box_free() and eqn_free() handle NULL.