WIP: no-alloc

[pinkforest]

Ref:

• no-alloc core-only "heapless" support #51
• no-alloc entropyxyz/crypto-primes#107
• [WIP] Fixed-size no_alloc-friendly keys #636
• Restore RNS functionality for keys with >2 primes #508
• no-alloc: What to include #639
• Rename MontyForm => FixedMontyForm crypto-bigint#1145
• HeaplessUint<const N: usize> crypto-bigint#1148

In preparation for no-alloc RSA we would need gating alloc to default-on alloc feature.

Anyone using this in no_std context with default-features == false needs to add alloc feature given the current impl is alloc.

But on top if this we can start adding generic no-alloc impl without breaking the current alloc-dependant impl.

I could start trying with FixedPrivateKey etc. on next PR and then we can generalize actual algos around that and alloc one without breakage.

Also need to add the alloc gating into crypto-primes and / or just avoid keygen for no-alloc to start.

[tarcieri]

I think something like this will make more sense when there's some level of baseline no_alloc functionality in-place, but right now we don't have any so gating everything like this will just leave an empty crate if you shut the alloc feature off

[pinkforest]

yea true. I'll leave this as WIP as it's required anyways for testing no-alloc impls and seeing what needs to get done.

anyone wanting to join the effort can just PR my branch