Don't allow unions to contain integer and double
Don't allow unions to contain integer and double

We unify integer and double to double, so the same
thing should happen in a union.  In addition to that,
we're using `oneOf` to represent unions in JSON
Schema, which requires that all the options are
mutually exclusive, which is broken by including
both integers and doubles.

quicktype/quicktype,quicktype/quicktype,quicktype/quicktype,quicktype/quicktype,quicktype/quicktype