Large changes checked in

[Ian Lance Taylor]

I checked in some large changes to the hash table, the scrubber, and
the symbol handling code.

The symbol handling changes are the worst.  These will probably break
most targets.  I checked and fixed many popular ones.  I'll fix the
rest over time.  If you run into trouble, let me know.  Failure is
indicated by a failure to compile.

The goal was to speed up gas on large files.  On a particular 27M
input file I was using as a test case, these patches change gas from
130.44+2.52 user+cpu time and 46M of memory to 44.85+1.65 and 34M.
Wall time on my system with 64M of real memory went from 2:42 to 0:53.

Please let me know about any problems.

Ian