Implement null? one of five rules of Scheme

Defines null? which is basic building block in Scheme.

* hy/core/language.hy: null? implementation
* docs/language/core.rst: document the null? in core
* tests/native_tests/core.hy: tests for null?

Signed-off-by: Vasudev Kamath <kamathvasudev@gmail.com>