RUST-1753: Add a utility HumanReadable wrapper

[abr-egn]

RUST-1753

I propose that instead of adding an explicit option to Collection to force on human-readable [de]serialization, we provide a type wrapper that does the same thing. This should provide equivalent functionality (instead of setting opts.human_readable_serialization = true for a given Collection<Foo>, specify the type as Collection<HumanReadable<Foo>>), but means that we don't have to support checking the explicit option in every relevant code path for every method. Subjectively, I also find this cleaner :)

I put this in the bson::serde_helpers module because it's arguably independent of the driver; it's even arguably independent of bson, but I don't think we want to maintain another crate.

If we do decide to use this, the path forward would be:

1. Put out a new release of the bson crate with these helpers.
2. Update the driver to remove the option and consistently use binary [de]serialization; AFAICT this is mostly done already as part of the earlier work, it just needs the fix in fixing the find or delete serde inconsistency mongo-rust-driver#948.
3. Put out a beta.1 release of the driver with those changes.

[isabelatkinson]

lgtm! i think we should add some docs/examples of how to use this in the driver but that can be part of step 2 :)

[abr-egn]

Unfortunately, writing tests for this shows that it doesn't work :( When serializing nested fields, a new serializer is constructed, and that doesn't have (and has no way of knowing about) the wrapper of the top-level serializer.