Let the user choose the underlying hash function

[liamsi]

It would be cool if iavl would allow the user to choose the underlying hash function used.

While several hash functions are benchmarked internally,

iavl/benchmarks/hash_test.go

Lines 25 to 30 in 4f7cdd7

	{"ripemd160", 64, crypto.RIPEMD160.New()},
	{"ripemd160", 512, crypto.RIPEMD160.New()},
	{"sha2-256", 64, crypto.SHA256.New()},
	{"sha2-256", 512, crypto.SHA256.New()},
	{"sha3-256", 64, crypto.SHA3_256.New()},
	{"sha3-256", 512, crypto.SHA3_256.New()},

only sha256 is really used and hardcoded in some places, e.g. see:

iavl/proof.go

Lines 56 to 57 in 5c3826b

	func (pin ProofInnerNode) Hash(childHash []byte) []byte {
	hasher := sha256.New()

or

iavl/node.go

Lines 200 to 205 in 7cab359

	func (node *Node) _hash() []byte {
	if node.hash != nil {
	return node.hash
	}
	h := sha256.New()

iavl/mutable_tree.go

Line 470 in a7d2e7b

existingHash = sha256.New().Sum(nil)

Ideally, the caller could pass in a hash function to use (for everything), or at least there would be a central place s.t. the hash can easily be changed (like with tendermint's tmhash).