hash: prepare for user-selectable hash functions

Localize the hash function in bees to a single spot to make it easier
to change later (or at runtime).

Remove some code that was using a property of CRC as an optimization.
The optimization doesn't work for other hash functions, and running the
CRC function takes more CPU time than the optimization saved.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees.h

@@ -605,6 +605,7 @@ struct BeesHash {
 	BeesHash(Type that) : m_hash(that) { }
 	operator Type() const { return m_hash; }
 	BeesHash& operator=(const Type that) { m_hash = that; return *this; }
+	BeesHash(const uint8_t *ptr, size_t len);
 private:
 	Type	m_hash;