merge merkle tree

- adding  PR for comments
I'm not sure here whether its necessary to wrap `mtree_store:lookup/2` error - is it possible to flatten this out, and "let it crash" ?

Annotating the return error atom with the module name is a good idea BTW - `mtree_hash_not_found` then exists in just one module in the codebase, and a simple grep when looking later for the module or function exception becomes very easy rather than a generic `not_found` which might be in OTP itself or our code.