Remove `multiply_by_rational`

[gavofyork]

Everything that uses it should instead be moved to use multiply_by_rational_with_rounding. SignedRounding should be used whenever inputs/outputs may be negative.

[tifecool]

I would like to attempt this issue if @Szegoo is dropping it.

Is SignedRounding a struct or function or will I just add my own logic to round the inputs/outputs?

[tifecool]

From my understanding, we want to work with integers for input, decimals within the function then, at output, round either up or down to the nearest integer.

Is this correct? @gavofyork

[Szegoo]

@tifecool You can have the issue :)

[tifecool]

Please see my prior comments @shawntabrizi @gavofyork