Use g_hash_table_new() rather than the _full() variant...

because we just default the other arguements anyway!
Rename hash_multiset -> hash_counting_table part 1
Rename hash_multiset -> hash_counting_table part 2
Improve hash_counting_table interface names
Merge branch 'multiset_improvements'

Pull request 1: https://github.com/foolswood/adifflib/pull/1