Don't obstack-allocate the call site hash table

The call site hash table is the last hash table using obstack
allocation.  In one large (non-public) test case, these hash tables
take a substiantial amount of memory.  Some of this memory is wasted
-- whenever the hash table is resized, the old table is not freed.

This patch fixes the problem by changing this hash table to be
heap-allocated.  This means that resizing will no longer "leak"
memory.