Retire support for nonstandard JSON syntax in atdgen 3.0

[mjambon]

From the beginning, atdgen -j has been producing nonstandard JSON for tuples (("a", 42) instead of ["a", 42]) and variants (<Foo> instead of "Foo", <Bar:[1,2]> instead of ["Bar", [1, 2]]). The -j-std option has been necessary to produce standard JSON but would become the default and support for the extended JSON syntax would disappear.

Yojson is getting rid of the feature, and since Atdgen relies on Yojson functions for this, it should be done in Atdgen as well, ideally before Yojson 3.0 is released.

This task is listed as part of the wishlist for atdgen 3.0 (#369). I don't think the version number matters much. Also, since all the packages of the atd suite use the same version number, we're not really following semver anymore (if we ever did).