Add support for sorted collections

Currently, it seems there is no support for sorted collections. Understandably, it was not a top priority since there are not that common.

They are not common but when needed, they become invaluable. Furthermore, since they are part of Clojure/script, it makes sense opening a discussion. I did not find anything, any issue on the matter. If you did consider supporting them and retracted for some reason, it would be useful providing a very brief summary here to at least have one issue documenting the process.

A few points:

0. It is usually disastrous when algorithms meant to operate on sorted collections blindly operate on unsorted ones (eg. a mistake, a bug, something escaped validation...)

1. Malli strives to be as serializable as possible. This can be problematic when using sorted-map-by and sorted-set-by which accept a custom sorting function. Using SCI in any way is not an option for performance reasons. However being serializable is not mandatory hence this is a minor problem. Furthermore, from experience, default sorted-map and sorted-set are often sufficient in most cases and those sorted default collections can be readily serializable.

2. Using custom sorted collections (eg. sorted-map-by) is akin to opening Malli to supporting any custom collection. A gateway to chaos?

3. Spec kind of eschews serialization but it almost solves this very simply by having an :into option in s/every and friends. Something like:

[:map-of
 {:into (sorted-map)}
 :int
 :string]

4. Example in Point 3 works for generation but not validation. Surprisingly, validation is tricky. You can check if a collection is sorted using sorted? but you never now if it is sorted as intended, returning back to Point 1. I don't think the sorting function can be reliably retrieved in both Clojure and CLJS. Users will have to resort to an alternative scheme such as adding a type in metadata if validation of custom sorted collections is needed.