Use fully-qualified syntax for ::BITS and prep for stdlib stabilization

[myrrlyn]

This commit only replaces each unqualified use of $type::BITS with <$type as crate::util::Integer>::BITS as usize, and marks the Integer::BITS constant as a u32.

This change allows core to stabilize the BITS constant on the integer primitives.

Once core stabilizes the constant, you can replace the definition from mem::size_of::<$t>() * 8 to <$t>::BITS. You will also be able to strip the < as crate::util::Integer>:: qualification syntax from most of the use sites, which use known typenames rather than trait-only generics.

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In a followup PR, if you assent, I will replace much of the Integer trait with my funty crate so that you do not need to have the maintenance burden of integer typing going forward.

[myrrlyn]

I didn't see #56 until after I wrote this. Closing in favor of that.