uint: 32-bit limbs are not supported properly

[nazar-pc]

construct_uint! {
    pub struct U64(2);
}

This piece of code generates some code that makes sense, like this:

    ///   Conversion to u32
    #[inline]
    pub const fn low_u32(&self) -> u32 {
        let &U64(ref arr) = self;
        arr[0] as u32
    }

But mostly non-sensical like this:

pub struct U64(pub [u64; 2]);

It'd be nice if it recognized that 64/2 == 32 and generated [u32; 2] instead.

The use case is SPIR-V target, which is technically 64-bit, but requires special capability in practice, which is not supported by some GPUs, so I need u64 that is a [u32; 2] and u128 that is [u32; 4].