[gcc 4.5 patch] Permit smaller hash tables for hash_map and hash_set

[Benjamin Kosnik]

> Do we have a general policy for patches to hash_map and hash_set, now
> that they have moved to include/backward?

Not really. You've made a reasonable case. I thought these two files
also had special status as they are symlinked to ext/hash_map and
ext/hash_set due to popular outrage. This is a bit of a mess.

> I have a few patches to make them better in some circumstances.  These
> ideas are incorporated in different from in the newer unordered_set
> and unordered_map.  However, existing code still uses hash_map and
> hash_set, so it may be useful to update them if we think that is OK.

I believe there was a previous attempt to fix up this stuff. It was not
well-received, although I don't especially remember the details. At this
point, I think this kind of stuff should just be checked in. 

-benjamin