Optimise serialization of MerklePath, avoiding ambiguity of std::vector<bool>

The serialization enforces a maximum supported Merkle tree depth of 64.

Closes #2831.

src/gtest/test_merkletree.cpp

@@ -40,16 +40,6 @@ void expect_deser_same(const ZCTestingIncrementalWitness& expected)
     // canonical serialized representation.
 }
 
-template<>
-void expect_deser_same(const libzcash::MerklePath& expected)
-{
-    // This deserialization check is pointless for MerklePath,
-    // since we only serialize it to check it against test
-    // vectors. See `expect_test_vector` for that. Also,
-    // it doesn't seem that vector<bool> can be properly
-    // deserialized by Bitcoin's serialization code.
-}
-
 template<typename A, typename B, typename C>
 void expect_ser_test_vector(B& b, const C& c, const A& tree) {
     expect_test_vector<B, C>(b, c);