Update usage of `*babyjub.PublicKey` to `babyjub.PublicKeyComp`
- when the key is not defined, internally is used `babyjub.EmptyBJJComp`, which is a `[32]byte` of zeroes of type `babyjub.PublicKeyComp`
- the API continues returning `nil` when the key is not defined

Add HashGlobalInputs for ZKInputs compatible with js & circom circuits version.

Compatible with hermeznetwork/commonjs at version: c6a1448db5
(c6a1448db5)

Refactor ZKInputs & add struct initialization & add BJJCompressedTo256BigInts util