Stabilize const for integer {to,from}_{be,le,ne}_bytes methods

All of these functions can be implemented simply and naturally as
const functions, e.g. u32::from_le_bytes can be implemented as

    (bytes[0] as u32)
        | (bytes[1] as u32) << 8
        | (bytes[2] as u32) << 16
        | (bytes[3] as u32) << 24

So stabilizing the constness will not expose that internally they are
implemented using transmute which is not const in stable.

src/libcore/lib.rs

@@ -131,7 +131,6 @@
 #![feature(rtm_target_feature)]
 #![feature(f16c_target_feature)]
 #![feature(hexagon_target_feature)]
-#![feature(const_int_conversion)]
 #![feature(const_transmute)]
 #![feature(structural_match)]
 #![feature(abi_unadjusted)]