* add accessors

* fmt

* comments

* add conversion for `SmtLeaf`

* introduce `SmtProof` scaffolding

* implement `verify_membership()`

* SmtLeaf: knows its index

* `SmtLeaf::index`

* `SmtLeaf::get_value()` returns an Option

* fix `verify_membership()`

* impl `SmtProof::get`

* impl `into_parts()`

* `SmtProof::compute_root`

* use `SmtProof` in `Smt::open`

* `SmtLeaf` constructors

* Vec

* impl `Error` for `SmtLeafError`

* fix std Error

* move Word/Digest conversions to LeafIndex

* `SmtProof::new()` returns an error

* `SparseMerkleTree::path_and_leaf_to_opening`

* `SmtLeaf`: serializable/deserializable

* `SmtProof`: serializable/deserializable

* add tests for SmtLeaf serialization

* move `SmtLeaf` to submodule

* use constructors internally

* fix docs

* Add `Vec`

* add `Vec` to tests

* no_std use statements

* fmt

* `Errors`: make heading

* use `SMT_DEPTH`

* SmtLeaf single case: check leaf index

* Multiple case: check consistency with leaf index

* use `pub(super)` instead of `pub(crate)`

* use `pub(super)`

* `SmtLeaf`: add `num_entries()` accessor

* Fix `SmtLeaf` serialization

* improve leaf serialization tests

* Smt::inner_nodes

* Add conversion Smt -> MerkleStore

* add docstring to `Smt`

* add to docstring

* fmt

* add `leaves()` method to `Smt`

* add `kv_pairs` functions

* rewrite `into_elements()` in terms of `into_kv_pairs()`

* change docstring

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