Use 16-bit lengths on 16-bit targets

[kornelski]

This changes the internal length type from u32 to u16 on 16-bit targets. On 16-bit targets larger arrays won't be used, 32-bit arithmetic can be expensive, and 4 bytes is a significant overhead.

target_pointer_width is not going to be smaller than "16", so #[cfg(not(target_pointer_width = "16"))] is sufficient to mean > 16.

Rustup doesn't ship a 16-bit target by default, so unfortunately it's not easy to add a compile test for it, but the tests pass with u16 LenUint on larger platforms.