Multiproof with tree trait (#44)

* Turn insert_leaf into self.insert + add tree trait

* First attempt at implementing the iterator

* Attempt at rewrite

* Rewrite iterator so that it works

* Move `trait Tree` to its own module
Also, set the number of children reported by Node::Branch to 16 instead
of the number of non-empty nodes.

* Remove insert_leaf and associated code duplication

* multiproof generation independent from tree type

* Fix test build

* Fix cargo build

* Fix unit tests

This commit effectively makes the parameter in LEAF() obsolete,
because it is assumed that the leaf only contains the label that
it needs.

Cargo.toml

@@ -9,8 +9,5 @@ edition = "2018"
 [dependencies]
 sha3 = "0.8.2"
 rlp = "0.4.2"
-hex = "0.3.2"
+hex = "0.4.0"
 serde_json = "1.0.40"
-
-[dev-dependencies]
-hex = "0.3.2"