Improve canonicalisation for conditional keywords

[Stranger6667]

I noticed that some conditional schemas are canonicalised very slow, consider this one:

SCHEMA = {
    "if": {
        "if": {"if": {"type": "null"}, "then": {"type": "null"}},
        "then": {"if": {"type": "null"}, "then": {"type": "null"}},
    },
    "then": {
        "if": {"if": {"type": "null"}, "then": {"type": "null"}},
        "then": {"if": {"type": "null"}, "then": {"type": "null"}},
    },
}

On my machine, each canonicalish call takes ~ 30ms, but if the "if" schema is equal to "then" and there is no "else" (or it is truthy), then this condition is a no-op.

It is slightly related to #64, where the example was not canonicalised to {"not": {}}, with this change it does, but it doesn't solve the general case with anyOf, though (as per your comment)

[Zac-HD]

Merged via f40063a - apparently github doesn't pick up on that since I rebased the conflict away 🤷‍♂️

As usual, thanks for the patch!