hash user allocates struct hash_table

hash user allocates struct hash_table

struct hash_table, a fixed-sized structure, is now allocated by the
caller.  This lets us integrate it into the Context structure, thus
avoiding an additional dynamically allocated object for no good
reason.

Add some minor code collapsing: make it more obvious that all that
differs is a pointer value, rather than relying on the compiler to do
tail merging.