9ded58fc99 Move merkle_tree hash types to primitives (Tobin C. Harding)
Pull request description:
In preparation for moving the `block::Header` struct over to `primitives` move the `merkle_tree` hash types.
ACKs for top commit:
apoelstra:
ACK 9ded58fc99 successfully ran local tests
Tree-SHA512: 98017cf0403871f01a6efeda22e8f319cc8104b9bc2f3a9bae2d6a31f6df8172307466c6486a9259204166933137fa03e565e08a0c156c278cfeb72cdae09b89