Try converting ad-hoc msymbol hash tables to STL containers

I went with multimap (also tried unordered_multimap) to keep
the behavior where we have a special hash key and want
to iterate over everything with that hash using specialized
compare functions.

Unfortunately this is a 10% regression.
real	0m56.538s
user	0m34.304s
sys	0m22.380s

real	0m51.655s
user	0m32.194s
sys	0m19.528s