Introduce scheme flavor of Hy.

As mentioned by @olasd in IRC, hy.flavor.scheme is born.

* hy/flavor/scheme.hy: null? function introduced
* docs/flavor/scheme.rst: Documentation for scheme flavor of Hy
* tests/native_tests/flavor/scheme.hy: Tests for scheme flavor of hy

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