Conjunct properties entries in a similar fashion to Items entries

[mniip]

Currently if we have an allOf of several schemas with items in them, we will sometimes convert it into a single items clause with an allOf inside. Should do the same with properties, though it's a bit more tricky considering their requiredness and presence of additionalProperties.

[mniip]

Thought: split the Properties condition into a conjunction of many conditions -- one for each property, and something for "everything else".