Move the `merkleblock` module into the `merkle_tree` module in a
submodule called `block`. In order to do the minimum amount of changes
in this patch DO NOT rename types to improved naming and reduce stutter.
Note:
- block module is private
- the three types are re-exported from `merkle_block`
This patch purposefully does the minimum amount of changes because there
a whole bunch of improvements to the old "merkleblock" module that are
coming next.
In preparation for moving `MerkleBlock` into the `merkle_tree` module;
create a new directory for the module and move `merkle_tree.rs` to
`merkle_tree/mod.rs`.
Tobin C. Harding