refactor(utils): enable the int_bits_const feature

`int_bits_const` feature [1] adds `i32::BITS`, etc. We already have
`BitInteger` trait, which has `BitInteger::BITS`, and using it triggers
`unstable_name_collision` compatibility lint [2] unless `int_bits_const`
is explicitly enabled, gets stabilized or gets rejected and removed from
the compiler.

[1]: rust-lang/rust#76492
[2]: rust-lang/rust#48919

Author: yvt

src/constance/src/lib.rs

@@ -13,6 +13,7 @@
 #![feature(unsafe_block_in_unsafe_fn)] // `unsafe fn` doesn't imply `unsafe {}`
 #![feature(never_type)] // `!`
 #![feature(specialization)]
+#![feature(int_bits_const)]
 #![feature(untagged_unions)] // `union` with non-`Copy` fields
 #![cfg_attr(test, feature(is_sorted))]
 #![deny(unsafe_op_in_unsafe_fn)]

src/constance/src/utils/ctz.rs

@@ -1,6 +1,5 @@
 //! Count trailing zeros
 #![allow(clippy::if_same_then_else)]
-use super::int::BinInteger;
 
 const USIZE_BITS: u32 = usize::BITS;
 

src/constance/src/utils/int.rs

@@ -96,7 +96,7 @@ macro_rules! impl_binary_integer {
         impl BinInteger for $type {
             type OneDigits = OneDigits<Self>;
 
-            const BITS: u32 = core::mem::size_of::<$type>() as u32 * 8;
+            const BITS: u32 = <$type>::BITS;
 
             #[inline]
             fn ones(range: ops::Range<u32>) -> Self {