Add type parameter to fromBase16

[aslesarenko]

The task is to generalize existing implementation of fromBase16 to allow embedding of arbitrary constants in ErgoScript sources.

val c = fromBase16[(Coll[Byte], Int)]("<base 16 string>")
c._1.size > 0 && x._2 > 0

Implementation hints:

• You need to implement PredefinedFunc("fromBase16", correctly (generalize existing implementation)
  See val DeserializeFunc = PredefinedFunc("deserialize",
  for an example of how to obtain SType value for the type T.
• The type T should already be parsed by the parser.
• there is already Base16 decoder, you can reuse is to get decodedBytes from String.
• Then having decodedBytes and tpe: SType for T you can use ConstantSerializer to parse the bytes to obtain a value of type T ( as instance of ConstantNode(value, tpe))