These conversion routines accept a `round` argument to control how the
result is rounded and return whether the result is exact. Most callers
wanted this functionality and had hacks around it being missing.
Also delete `std.math.big.rational` because it was only being used for
float conversion, and using rationals for that is a lot more complex
than necessary. It also required an allocator, whereas the new integer
routines only need to be passed enough memory to store the result.
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