Float-to-fixed point stake calculations

[grod220]

A draft of the C implementation of new fixed point helpers from SIMD-0391.

Meant to mirror the work done in Rust: https://github.com/solana-program/stake/pull/152

Note: It is not integrated into the core client calculations. It adds the new fixed-point helpers independently, but does not add the necessary feature gating to be ready. Feature gating needed in:

• Runtime & stake logic for stake activation and deactivation calculations
• Stake cache & history for effective stake derivation and history aggregation
• Inflation rewards for calculation of stake-weighted rewards

[topointon-jump]

Thanks for this! Creating a Firedancer PR for a SIMD is awesome 🙌

Happy to get this PR across the line once the SIMD is approved 😄