Software div_euclid refactoring.

`f{16,32,64,128}/soft.rs` are now very similar. f32 and f64 almost
identical.

`U256` helper moved to `crate::u256`.

`(-5f32).div_euclid(f32::INFINITY)` now returns -1 not 0.

Author: tczajka

library/std/src/f128.rs

@@ -4,8 +4,7 @@
 //!
 //! Mathematically significant numbers are provided in the `consts` sub-module.
 
-mod div_euclid;
-mod u256;
+mod soft;
 
 #[cfg(test)]
 mod tests;
@@ -271,7 +270,7 @@ impl f128 {
     #[unstable(feature = "f128", issue = "116909")]
     #[must_use = "method returns a new number and does not mutate the original value"]
     pub fn div_euclid(self, rhs: f128) -> f128 {
-        div_euclid::div_euclid(self, rhs)
+        soft::div_euclid(self, rhs)
     }
 
     /// Calculates the least nonnegative remainder of `self (mod rhs)`.