Deal with _apply_iterate

[vchuravy]

Fixes #155 and #146

cc: @YingboMa @shashi

[KristofferC]

Does this do the:

Cassette and similar tools
can override this argument and provide a function that properly
allows the context to recurse through the iteration, while still
allowing inference to take advantage of the special handling of _apply
for simple cases.

[vchuravy]

can override this argument and provide a function that properly
allows the context to recurse through the iteration,

Yes it does, it rewrites the argument with a args -> overdub(ctx, f, args...)

[shashi]

Awesome!