Convert linespec.c to new hash table

Convert linespec.c to new hash table

This converts linespec.c to use the new hash table.

Note that more simplification could perhaps be done.  Currently, the
collectors in this code insert an element into a set and then, if the
element has not been seen before, append it to a vector.  If we know
the order does not matter, or if the results can be sorted later, we
could dispense with the vector.  This would simplify the code some
more.  (This technique is used in the vtable patch, later in this
series.)

Change-Id: Ie6828b1520d918d189ab5140dc8094a609152cf2
Co-Authored-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>