Exposes approximate_float for unsigned ratios

[nullsauce]

Hey there. Cool crate!

Any reason approximate_float_unsigned isn't exposed in the lib interface ? It makes it impossible to approximate into unsigned ratios. Here's a small patch that fixes that. Hope it helps!

[cuviper]

For primitive floats you can use FromPrimitive::from_f32 or from_f64, but for generic FloatCore we do need something else. I guess this was just an oversight, and nobody had asked for it yet. Thanks!

bors r+

[bors[bot]]

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[cuviper]

bors r+

[bors[bot]]

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