Reorganize core::num internals

Move private bignum module to core::num, because it is not only used in flt2dec.
Extract private 80-bit soft-float into new core::num module for the same reason.

src/libcoretest/num/bignum.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 use std::prelude::v1::*;
-use core::num::flt2dec::bignum::tests::Big8x3 as Big;
+use core::num::bignum::tests::Big8x3 as Big;
 
 #[test]
 #[should_panic]

src/libcoretest/num/dec2flt/rawfp.rs

@@ -9,14 +9,14 @@
 // except according to those terms.
 
 use std::f64;
-use core::num::flt2dec::strategy::grisu::Fp;
+use core::num::diy_float::Fp;
 use core::num::dec2flt::rawfp::{fp_to_float, prev_float, next_float, round_normal};
 
 #[test]
 fn fp_to_float_half_to_even() {
     fn is_normalized(sig: u64) -> bool {
-            // intentionally written without {min,max}_sig() as a sanity check
-            sig >> 52 == 1 && sig >> 53 == 0
+        // intentionally written without {min,max}_sig() as a sanity check
+        sig >> 52 == 1 && sig >> 53 == 0
     }
 
     fn conv(sig: u64) -> u64 {

src/libcoretest/num/flt2dec/mod.rs

@@ -23,7 +23,6 @@ use core::num::flt2dec::{to_shortest_str, to_shortest_exp_str,
 pub use test::Bencher;
 
 mod estimator;
-mod bignum;
 mod strategy {
     mod dragon;
     mod grisu;

src/libcoretest/num/flt2dec/strategy/dragon.rs

@@ -12,7 +12,7 @@ use std::prelude::v1::*;
 use std::{i16, f64};
 use super::super::*;
 use core::num::flt2dec::*;
-use core::num::flt2dec::bignum::Big32x40 as Big;
+use core::num::bignum::Big32x40 as Big;
 use core::num::flt2dec::strategy::dragon::*;
 
 #[test]

src/libcoretest/num/mod.rs

@@ -31,6 +31,7 @@ mod u64;
 
 mod flt2dec;
 mod dec2flt;
+mod bignum;
 
 /// Helper function for testing numeric operations
 pub fn test_num<T>(ten: T, two: T) where