Factor-out GenericsCapableSignatureReader from TypeInvoker.

[felixvf]

The TypeInvoker is actually misused as a GenericsCapableSignatureReader, doing functionality of both type incoking and reading signatures, but in both cases not performing that functionality perfectly, especially in this method:

  def saveTypeParam(var)
    typeVariables[var.toString] = @args.removeFirst unless @args.isEmpty
    @typeParams.add(var)
  end

Hence, the old TypeInvoker performing 2 distinct functions should be split into the new TypeInvoker and a GenericsCapableSignatureReader.