add support for architectures with 16-bit pointer width Closes #352

[mutantbob]

One example is AVR-based microcontrollers like the atmega328 or atmega2560

[mutantbob]

This is my attempt at fixing #352 . Now to fix the ahash crate...

[Amanieu]

LGTM, but you need to run cargo fmt.

[mutantbob]

Closes #352

[Amanieu]

After thinking about this for a bit, this may not be the right thing to do here. Using a group size that is too small will have a negative impact on workloads which frequently insert & remove elements since it makes it more likely to completely fill a group with tombstones. It might be better to use u32 like the 32-bit architectures here.

Another issue is that AVR (RISC-V too) doesn't support an efficient implementation of u32::trailing_zeros and unaligned memory access which are crucial for the performance of the hash table. This can probably be looked over for now but in the future we may want a different implementation for these architectures.

[Amanieu]

ping @mutantbob Can you change this PR to use u32 on 16-bit architectures?

[Amanieu]

Superseded by #368