fixed_point_t uses multiplication/division instead of arithmetic shift

[salkinium]

For constexpr this doesn't really matter, however, arithmetic shift is probably faster.

There is a subtle difference between v >> s and v / (1 << s):

One important consequence of the arithmetic right shift is that it will round negative
numbers towards negative infinity while a division is always rounding towards zero. So
a right shift on negative numbers is not identical to a division.

Example: -3 / 2 = -1
-3 >> 1 = -2

(taken from this discussion on the topic)

So the eps and cmath rounding algorithms will have to be adapted.