[core] Optimize dynamic storage allocator (#472)

* [core] Fix typos

* [core] Add BitStash::is_completely_full()

* [core] Split logic and get rid of loop

* [core] Add benchmarks for storage2::BitStash

* [core] Shorten code

* [core] Remove outcommented code

* [core] Get rid of warnings

* [core] Apply cargo fmt

* [core] Remove unnecessary mutable

* [core] Remove unnecessary check

* [core] Add more storage2::BitStash benchmarks

* [core] Fix benches (panics for empty_cache::one_put)

* [core] Fix empty_cache::one_put bench

* Revert "[core] Remove outcommented code"

This reverts commit 98a74324.

* [core] Improve benchmark labels

* [core] Use iter_batched_ref consistently

* [core] Remove unnecessary black_box

* Revert "Revert "[core] Remove outcommented code""

This reverts commit aaf24713.

* [core] Satisfy clippy
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