Add existence helpers to cw-storage-plus

[ethanfrey]

Inspired by #251 (comment)

We should add a quick existence operator to Map / IndexedMap that doesn't parse the data. This could be used like load:

let exists: bool = TOKENS.has(deps.storage, &token_id);

We should also expose this on the Path helper (which would be how the above is implemented`

TOKENS.key(k).exists(deps.storage);

Note that neither must return errors (like remove) as neither parses data