The type for keys.

The equality function for keys.

Note: Unevenly distributed hash functions may result in performance drops.

The number of bits necessary to encode the maximum output value of hash. `Hashtbl.hash` uses 30 bits.

Overestimating the hash_size will result in performance drops; underestimation will result in undefined behavior.

encode is an encoding function. The resultant encoded values must have size encoded_size.

encoded_size is the size of the result of encode, expressed in number of bytes.

decode s off is the decoded form of the encoded value at the offset off of string s. Must satisfy decode (encode t) 0 = t.