Benchmark integer `Key` performance

[maurolacy]

In CosmWasm/cw-plus#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.

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.

[uint]

We now know longer keys impact performance significantly - I doubt benchmarking this is worth the effort. The effect has been documented in the Guide.