Enable fixed point u128 (#6214)

* Add fixed u128.

* remove move

* Change sat_from_integer impl.

* checked_pow is always positive

* Revert.

* rename fixed file

* Rename to FixedI

* rename fixed file

* Add newline.

* Use Multiplier in impls.

* Renames negate() to saturating_negate().

* Uncomment test.

* Add Signed to macro.

* Add some tests for Saturating trait.