refactor: Small-field function parameters

[Tabaie]

With fields where the whole element fits in one word, there is no upside to passing function arguments by reference. For example func (z *Element) Add(*Element, *Element) *Element is best turned into func (z *Element) Add(Element, Element) *Element.

This would of course cause a divergence in the API for small and large fields and possibly make writing generic code more difficult, but since in any cryptographic application small field elements need to be bunched together, different code needs to be written for the two cases anyway.