hy/core/macros.hy: Add an (if-not) macro

Sometimes it is better to start with the false condition, sometimes that
makes the code clearer. For that, the (if-not) macro, which simply
reverses the order of the condition blocks, can be of great use.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
hy/core/macros.hy: Reimplement (unless) in terms of (if-not)

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>