Remove {uint,int,u64,i64,...}::from_str,from_str_radix

Remove these in favor of the two traits themselves and the wrapper
function std::from_str::from_str.

Add the function std::num::from_str_radix in the corresponding role for
the FromStrRadix trait.

With `from_str` in the prelude, and `from_str_radix` in `std::num`, the feature is unfied under the type annotation of these functions instead of using the modules-named-as-types (std::uint and others):

What was before:

    let n = std::uint::from_str("1");
    let m = std::i32::from_str_radix("10", 16);

is now:

    let n = from_str::<uint>("1");
    let m = std::num::from_str_radix::<i32>("10", 16);