Upodate idl to hni converter

Summary: We now handle variadics, paramcoercenull,
and the correct foldable flag.

Reviewed By: @JoelMarcey

Differential Revision: D1519934
Rewrite misc.idl.json to HNI

Summary: This pull request moves the functions in misc.idl.json to HNI.

Part of #1480
Closes https://github.com/facebook/hhvm/pull/3525

Reviewed By: @fredemmott

Differential Revision: D1506187

Pulled By: @sgolemon