feat: Add PrimaryKey implementation example for custom types

[bullet-tooth]

Why?

Sometimes, it's useful to store a custom type as a key in a Map storage structure. To do this, users need to implement proper serialization for such types.

So I found it useful to add such an example with a reference implementation for a widely used Denom enum that represents either a native token or a cw20 token.

Scope:

• Made some util functions as the public that can be helpful in custom types implementations
• Added example test with primary key implementation for `Denom
• Added a readme reference for that example

[bullet-tooth]

Question for maintainers: From the selectivity perspective, mb, is it better to write the value first as a prefix and after its type (which is either 1 or 2 in our case)?

[uint]

That would cause iteration order to be very unintuitive. I suspect prefixing is better.

[uint]

Hi! It's definitely a good idea to document primary key implementation. Thank you for doing this and for your patience.

A much better place for this would be our official guide:

https://docs.cosmwasm.com/cw-storage-plus/containers/map
https://github.com/CosmWasm/docs

Would you be open to porting the documentation work there? Otherwise, if you reduce the scope of this PR to just making the helper functions public, I can take it over at some point and add the documentation to the guide.

[bullet-tooth]

@uint sure, thanks for your comments.
I will make a PR to the guide next week.

By the way, it would be great to review my implementation because I'm not 100% sure it's optimal and production-ready.

[uint]

As far as I understand, if we split these two elements as separate "subkeys", your prefix will end up length-prefixed itself.

It might be better to return a one-element vector here, combining the prefix with the value by hand. Then other methods would need to be adjusted accordingly and KeyDeserialize::KEY_ELEMS would have to be 1, I think.