The store builds the path from root to leaf, this updates the code to
return a path from leaf to root, as it is done by the other structures.

This also added custom error for missing root.

Prior to this commit, the MerkleStore panicked under certain bounds. It
will prevent such panics by using checked operations.

ilog2, for instance, will panic when the operand is zero. However, there
is a documentation rule enforcing the merkle tree to be size at least 2.
If this rule is checked, then the panic is impossible.

This commit introduce common derive proc macros to MerkleStore. These
are required downstream as the in-memory storage can be cloned.

It also introduces constructors common to the other types of the crate
that will help to build a merkle store, using a build pattern.