Change prefixes used by the Float trait

Change `EXPONENT_` to `EXP_` and `SIGNIFICAND_` to `SIG_`. These are
pretty unambiguous, and just makes for less to type once these get used.

library/compiler-builtins/libm/crates/libm-test/src/mpfloat.rs

@@ -15,7 +15,7 @@ use crate::Float;
 
 /// Create a multiple-precision float with the correct number of bits for a concrete float type.
 fn new_mpfloat<F: Float>() -> MpFloat {
-    MpFloat::new(F::SIGNIFICAND_BITS + 1)
+    MpFloat::new(F::SIG_BITS + 1)
 }
 
 /// Set subnormal emulation and convert to a concrete float type.