Closes #254. Besides Error also applies to Ok, coming from Ppx_deriving_runtime.

Conceptually the fix is simple: just add a type constraint to the to_int function generated into the wildcard_case of variant comparison. This forces type-directed construction resolution to correctly use the constructor from our type instead of Result.

The tricky part was not breaking other tests, where a new variant type is defined with the same name as a standard one, e.g. bool. In that case the to_int type constraint inside the comparator must still refer to the outer bool type being declared, not the one from Ppx_deriving_runtime, which happens to be opened at that point due to sanitize.
A local module definition Ppx_deriving_ord_helper is used to capture the type being declared outside of sanitize, such that inside it, we can refer to the correct bool. Hopefully such type-only local module doesn't have any runtime cost (?), otherwise I'll have to make it conditional to only happen for variant types (currently it's generated unconditionally and otherwise just unused).