Add functions to safely transmute float to int

src/libstd/lib.rs

@@ -321,6 +321,7 @@
 #![feature(zero_one)]
 #![cfg_attr(test, feature(update_panic_count))]
 #![cfg_attr(stage0, feature(pub_restricted))]
+#![cfg_attr(test, feature(float_bits_conv))]
 
 // Explicitly import the prelude. The compiler uses this same unstable attribute
 // to import the prelude implicitly when building crates that depend on std.