Generate Conversion<V> implementations to return literals

[iliekturtles]

Generate impls using some kind of compile-time calculation: custom derive, build.rs, const fn? Currently all conversion factors go through f64 which may have run-time costs and causes imprecision issues where the conversion factor can be exactly represented as Self::T but not as f64. Also see #40 regarding conversion factors that can't be represented.

For example the Conversion<V> implementation for primitive integer and BigInt types uses Self::T::from_f64:

fn conversion() -> Self::T {
    Self::T::from_f64($conversion).unwrap()
}

Ideally the method should be generated to return a pre-calculated literal for each type:

fn conversion() -> Self::T {
    Self::T::new_raw(10, 1) // assuming Self::T is Ratio<V>.
}