Convert more print statements in the gdb .py files to run under python 3...

Summary: ....

This follows:
  https://github.com/facebook/hhvm/commit/fa214a8f475a66ab66fa2778af4019e7a298499c
This addresses issue:
  https://github.com/facebook/hhvm/issues/3448
Closes https://github.com/facebook/hhvm/pull/3569

Reviewed By: @ptarjan

Differential Revision: D1517079

Pulled By: @sgolemon
Move var_dump() to HNI, kill off variable.idl.json

Summary: Note that this modifies xdebug code-coverage behavior.

Our xdebug implementation doesn't profile IDL based
builtins, but it does happily profile HNI based ones.

It *should* profile all IDL based functions,
so that'll have to be fixed in a followup,
but for now I've re-recorded the effected xdebug tests
to provide the correct results.

Reviewed By: @ptarjan

Differential Revision: D1515818