Handle remaining keywords when merging schemas

[Zac-HD]

https://github.com/Zac-HD/hypothesis-jsonschema/blob/c16b93bde5616fa4ecbe2808d7ba9fe7a221faf4/src/hypothesis_jsonschema/_canonicalise.py#L408-L420

The merged(schemas) function outputs a single schema which matches all and only instances matched by all of the input schemas, or None if there is no such schema (without e.g. allOf).

This issue will be closed when merged() understands how to merge all keywords defined in the spec that can in principle be merged. This includes e.g. maximum (take the min), but not contains (at least one array item valid against each)

[Zac-HD]

• multipleOf - calculate least common multiple of integer-valued arguments. Non-integer values are not going to be supported though, I don't want to think about the floating-point issues.
• uniqueItems - make canonicalish drop "uniqueItems": false from schemas and the existing merge logic will be sufficient.

In both cases we'll be able to merge schemas that we couldn't before, and test appropriately. And that's the last two keywords!

[Zac-HD]

Closed by #44.