NewMerkleTree is a memory hog, we can do better than that:
BenchmarkMerkle/NewMerkleTree-8 13 88434670 ns/op 20828207 B/op 300035 allocs/op
BenchmarkMerkle/CalcMerkleRoot-8 15 69264150 ns/op 0 B/op 0 allocs/op
C# node is quite picky as it expects there to be exactly one value returned,
but our testchain actually adds 4 signatures for multisig cases instead of 3
which makes it technically incompatible with C# node.
We make it explicit in the appropriate Block/Transaction structures, not via a
singleton as C# node does. I think this approach has a bit more potential and
allows better packages reuse for different purposes.
Our test chain is used in core,rpc and (in future) interop packages.
It is better to have all related declarations in one place to avoid
code duplication.
