Introduce SmtProof (#270)

* 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

src/merkle/smt/simple/mod.rs

@@ -1,5 +1,5 @@
 use crate::{
-    merkle::{EmptySubtreeRoots, InnerNodeInfo, ValuePath},
+    merkle::{EmptySubtreeRoots, InnerNodeInfo, MerklePath, ValuePath},
     EMPTY_WORD,
 };
 
@@ -302,4 +302,8 @@ impl<const DEPTH: u8> SparseMerkleTree<DEPTH> for SimpleSmt<DEPTH> {
     fn key_to_leaf_index(key: &LeafIndex<DEPTH>) -> LeafIndex<DEPTH> {
         *key
     }
+
+    fn path_and_leaf_to_opening(path: MerklePath, leaf: Word) -> ValuePath {
+        (path, leaf).into()
+    }
 }