Can't compile on 32-bit

[singron]

E.g. I was cross compiling for armv7-unknown-linux-gnueabihf.

error[E0284]: type annotations required: cannot resolve `<f64 as conv::ApproxFrom<usize, _>>::Err == _`
   --> src/quantile.rs:138:29
    |
138 |         let desired_index = f64::approx_from(len).unwrap() * self.p() - 1.;
    |                             ^^^^^^^^^^^^^^^^

As far as I can tell, this is since usize is really a u32 on 32-bit platforms, which has the Wrapping ApproxScheme in addition to DefaultApprox.