Merge overlapping items subschemas

[Zac-HD]

In hypothesis_jsonschema._canonicalise.merged(), we should calculate the intersection of items schemas.

This is complicated somewhat by the possibility that one or both schemas to merge may have a list-of-items-schemas and an additionalItems schema, or a single schema for all items. Fortunately we have analogous logic for object properties, so it's more fiddly than conceptually challenging.