Use a hash to keep exactly one copy of each filename around

Use a hash to keep exactly one copy of each filename around

The old code for keeping track of source file name and line was
confused as hell about ownership of the strings, and it is pretty
clear we leaked that information all over the place.

Instead, use a hash table to keep a copy of each string as necessary,
and simply make references to a string pool that we keep until the end
of the assembly session.

This pool probably should be unified with the list of dependency
files, and so on, but that is for the development branch.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>