mssmt: use Node interface instead of LeafNode concrete type in func sigs

Replaced usages of the LeafNode concrete type with the more general
Node interface in function signatures where applicable. This adds
flexibility which will become useful in a future itest.

Author: ffranr

mssmt/proof.go

@@ -42,7 +42,7 @@ func NewProof(nodes []Node) *Proof {
 }
 
 // Root returns the root node obtained by walking up the tree.
-func (p Proof) Root(key [32]byte, leaf *LeafNode) *BranchNode {
+func (p Proof) Root(key [32]byte, leaf Node) *BranchNode {
 	// Note that we don't need to check the error here since the only point
 	// where the error could come from is the passed iterator which is nil.
 	node, _ := walkUp(&key, leaf, p.Nodes, nil)

mssmt/tree.go

@@ -145,10 +145,10 @@ func (t *FullTree) walkDown(tx TreeStoreViewTx, key *[hashSize]byte,
 
 // walkUp walks up from the `start` leaf node up to the root with the help of
 // `siblings`. The root branch node computed is returned.
-func walkUp(key *[hashSize]byte, start *LeafNode, siblings []Node,
+func walkUp(key *[hashSize]byte, start Node, siblings []Node,
 	iter iterFunc) (*BranchNode, error) {
 
-	var current Node = start
+	var current = start
 	for i := lastBitIndex; i >= 0; i-- {
 		sibling := siblings[lastBitIndex-i]
 		var parent Node