Added Pow trait and implementations for base numeric types and usize.

[orizi]

…ze`.

[reviewable-StarkWare]

This change is 

[TomerStarkware]

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ArielElp and @gilbens-starkware)

[TomerStarkware]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Arcticae, @ArielElp, @Draggu, @gilbens-starkware, @mkaput, and @piotmag769)

[gilbens-starkware]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @Arcticae, @ArielElp, @Draggu, @mkaput, and @orizi)

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corelib/src/num/traits/ops/pow.cairo line 27 at r2 (raw file):

            } else {
                tail_result * self
            }

Consider changing to an iterative implementation, it is a bit more readable.
And if not, make exp and self immutable.

Suggestion:

            let mut res =  core::num::traits::One::one();
            let mut cur_multiplier = self;
            while exp != 0 {
                let (exp, rem) = DivRem::div_rem(exp,2);
                if rem == 1 {
                    res *= cur_multiplier;
                }
                cur_multiplier = cur_multiplier * cur_multiplier; 
            }

[orizi]

Reviewable status: 4 of 5 files reviewed, 1 unresolved discussion (waiting on @Arcticae, @ArielElp, @Draggu, @gilbens-starkware, @integraledelebesgue, @mkaput, and @TomerStarkware)

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corelib/src/num/traits/ops/pow.cairo line 27 at r2 (raw file):

Previously, gilbens-starkware (Gil Ben-Shachar) wrote…

Consider changing to an iterative implementation, it is a bit more readable.
And if not, make exp and self immutable.

that was my original implementation - but this solution overflows in cases when no overflow may occur.
(because of the squaring happening at the end of the loop).
mut removed.