feat: make solana-inflation crate no_std compatible

[sonicfromnewyoke]

Problem

solana-inflation crate is no_std-friendly but not really no_std compatible

the main obstacle in making this crate compatible with no_std environment is powf() method on f64 type which doesn't exist.

Proposed Solution

import num_traits::Float trait, which is accessible in libm feature.

Alternatives

1. import libm directly
2. copy/paste only powf() function from libm crate, and use it instead