hy/core/macros.hy: Add defn-alias / defun-alias

In the same vein as defmacro-alias, this implements defn-alias /
defun-alias, which does essentially the same thing as defmacro-alias,
but for functions.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge branch 'master' into pr/456