formatting fix for null? documentation

Signed-off-by: Vasudev Kamath <kamathvasudev@gmail.com>
include flavor.scheme tests in __init__.py for native_tests

Signed-off-by: Vasudev Kamath <kamathvasudev@gmail.com>
Add pair? function and fix logic of null? function

* docs/flavor/scheme.rst: Add documentation for pair? function
* hy/flavor/scheme.hy: Fix logic of null? function and add pair?
  function
* tests/native_tests/flavor/scheme.hy: Add tests for pair? function and
  replace assert-true and assert-false with assert statements.

Signed-off-by: Vasudev Kamath <kamathvasudev@gmail.com>