auto merge of #9275 : blake2-ppc/rust/float-from-str, r=thestinger

std: Remove {float,f64,f32}::from_str in favor of from_str in the prelude

Like issue #9209, remove float::{from_str, from_str_radix} in favor of
the two corresponding traits. The same for modules f64 and f32.

New usage is:

    from_str::<float>("1.2e34")