Canonicalise `oneOf` sub-schemas to exclude shared types

[Zac-HD]

Consider the schema {"oneOf": [{"required": ["foo"]}, {"required": ["bar"]}]}

What values could match this? Because the two sub-schemas only constrain objects, all values of any other type will be valid against both sub-schemas... and therefore invalid against the oneOf schema.

This tends to result in a lot of filtering, so we have a workaround in our tests for this test case.

Obviously it would be nicer to support such schemas properly, by canonicalising them to include type: object (i.e. exclude other types) and therefore translate them to a Hypothesis strategy which doesn't have to filter out so much.

The semantics are a bit too tricky for me to tag this good first issue, though the tests will catch any plausible mistakes you could make. We have some prior art in the handling of "not" sub-schemas, which might provide a good starting point; you might even move shared logic to a new helper function.

Alternatively, 'lowering' oneOf: [a, b, c] to anyOf: [{**a, not: {anyOf: [b, c]}}, ...] would automatically share any improvements to not-logic, which would be nice. Just a question of balancing that against the less elegant canonical form.

[Zac-HD]

I'm now convinced that 'lowering' is the way to go, along with associated improvements to canonicalising anyOf lists (e.g. de-nesting them, taking unions of schemas, etc.) and special handling for not: anyOf: ... patterns. Fortunately these are independently useful changes, though obviously complementary!