Expand array sizes through U1024

[tarcieri]

Post-quantum schemes like Kyber/Saber require sizes larger than what is currently supported.

This extends the allowed array sizes through U1024 in multiples of 16, although that still won't be enough to cover all cases.

However, it's the current limit of what typenum provides convenient type aliases for.

[tarcieri]

This causes a fairly significant increase in build times:

• Before: 2s
• After: 2m50s

For some reason I don't see this on a local build, though?

It would seem prudent to adopt only multiples of some power of two for these larger sizes, like I suggested in #45

Edit: this is actually a difference between 1.65 and stable, which explains why it built so fast locally. It looks like it currently takes 3m to build on 1.65: https://github.com/RustCrypto/hybrid-array/actions/runs/7923269699/job/21632628872

Edit again: opened #47 to track this.

[tarcieri]

Using multiples of 16 for sizes above 256 has reduced build times to 2s again, although we'll still need a solution for sizes >1024.