Implement beef::lean::Cow on other pointer widths

[Waelwindows]

This should remove the restriction on beef::lean::Cow being only available on targets with a pointer width of 64

[CAD97]

This is, strictly speaking, a breaking change, since on 32 bit targets lean::Cow switches from supporting u32::MAX elements to u16::MAX elements. (On 16 bit pointer targets, it goes from u16::MAX to u8::MAX elements. Ignoring 16 bit pointers from hereon.)

There's advantages to both ways. With usize lean::Cow, you always fit the cow into 2×usize. With u64 lean::Cow, the cow always supports u32::MAX elements, which is enough for (nearly) anyone, since you'll (typically) run out of RAM first.

lean::Cow is more necessary on 64 bit architectures, where pointers are thicc quite large comparatively. It's reasonable to use lean::Cow so save the space on 64 bit targets by restricting the capacity as if it were a 32 bit target, but still wanting to keep the extra space on 32bit (which is still less than 64bit lean) to keep the same max capacity.

I think there is value in exposing the 2×usize version to 32 bit architectures, but it should be under a new reëxport (beef::leaner::Cow? skim::Cow?), and documentation would need to be made extra clear so that it's clear from both 64bit and 32bit docs what the behavior is on both targets.