Map_like.HASHTBLGeneric hashbtl interface.
val create : int -> tcreate n is an empty hash table.
fold f h init folds over the bindings of h with f starting with init.
equal veq h0 h1 tests whether h0 and h1 contain the same keys and associate them with equal values according to veq
compare kcmp vcmp h0 h1 totally order h0 and h1 with kcmp used to compare keys and vcmp used to compare the value of equal keys.