Auto merge of #119430 - NCGThompson:int-pow-bench, r=cuviper

Add Benchmarks for int_pow Methods.

There is quite a bit of room for improvement in performance of the `int_pow` family of methods. I added benchmarks for those functions. In particular, there are benchmarks for small compile-time bases to measure the effect of  #114390. ~~I added a lot (245), but all but 22 of them are marked with `#[ignore]`. There are a lot of macros, and I would appreciate feedback on how to simplify them.~~

~~To run benches relevant to #114390, use `./x bench core --stage 1 -- pow_base_const --include-ignored`.~~

Author: bors