fixes: #258

* tests: fix typos
* full.rs: fix typo
* CONTRIBUTING: fix typo

* recompute_nodes_from_indeX_to_root

* MerkleError variant

* set_subtree

* test_simplesmt_set_subtree

* test_simplesmt_set_subtree_entire_tree

* test

* set_subtree: return root

When a prefix is pushed to the depth 64, the entry list includes only
the values different than ZERO. This is required, since each block
represents a 2^192 values.

The bug was in the proof membership code, that failed to handle the case
of a key that was not in the list, because the depth is 64 and the value
was not set.

* Change InvalidNumEntries error

* max computation

* remove length check

* remove ExactSizeIterator constraint

* fix InvalidNumEntries error condition

* 2_usize

* with_contiguous_leaves

* test