impl SIMD-0529: big_int_mod_exp syscall api refactoring

[zz-sol]

• this PR refactors the API to allow for both big and little endian.
• i also thought about the backend changes. we have the following options

library	2048-bit	4096-bit
crypto-bigint	3.34 ms	27.6 ms
num-bigint	2.15 ms	18.4 ms
rug	1.25 ms	8.49 ms

• crypto-bigint is constant time but prob not required
• num-bigint is currently being used
• rug is the fastest -- but it depends on gmp and cc (wrapped by rust) so there maybe some additional system requirement.

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follow up: rug requires LGPL-3.0+ license. we prob don't want to use it. so we stick with num-bigint.

[samkim-crypto]

Yeah in this case, let's stick with num-bigint. The changes looks good to me overall. Let's wait on merging this until solana-foundation/solana-improvement-documents#529 is merged. I do have some comments regarding the syscall function signature, but I commented on the SIMD side.