speedup in tag lookup using hash tables

Hi Ilpo,

	Today I optimized the dwarves a bit by using a per object file
hash table for tag lookup, it yelded almost 50% speedup on pahole when
running on a vmlinux file 

	codiff doesn't makes that much tag lookups, so we didn't got
much improvements there, but I'm doing experiments on dead tag elimination
that probably will help a lot there, but for that I have first to grok
Ulrich Drepper's libdisasm to find out what are the tags that are really
used by looking at accesses to register indexed memory areas that use as
a base pointer what is in local/global variables and function
parameters.

	This also will provide the basis for detecting access patterns
that will ultimatelly allow libdwarves_reorganize to do struct
reorganizations to improve locality of reference, etc.

	Please take a look at v1.6 that I pushed today and tell me your
impressions.

Regards,

- Arnaldo