Finite field elements can be in regular or Montgomery form, and
chaining different operations use to require manual and error-prone
conversions.

Now:

- `add`, `sub` and `mul` convert the second operand to match the
first operand's form
- `sq` and `pow` preserve the input's Montgomery form
- `toPrimitive` and `toBytes` return `UnexpectedRepresentation` if
the element is in Montgomery form, preventing incorrect serialization

This is fully backwards compatible and allows seamless chaining of
operations regardless of their representation.