Benchmark integer `Key` performance

In https://github.com/CosmWasm/cw-plus/pull/694 support for 128 bits integer keys was added.

There are some concerns with maximum integer key length, as [it impacts all the int types](https://github.com/CosmWasm/cw-plus/pull/547#discussion_r749359057).

Confirm this doesn't impact performance of reading / writing int keys to storage. It may be tricky to test / benchmark this, as code changes will be needed, to have a `Key` implementation with and without `u128` support. This can be a one time benchmark, in a branch that we end up not merging to main, by example.

Review PR 694 and the PR 547 discussion first, then locate the storage read and write paths for integer keys. Compare benchmarks for Key implementations with and without u128 support; done means reporting whether maximum key length affects integer-key storage performance.

Written by the indexing model from the issue text.