Remove the import-as and import-from builtins

The new and improved (import) can handle all cases import-as and
import-from did, so drop the latter two from the language. To do this,
the import builtin had to be changed a little: if there's a single
import statement to return, return it as-is, otherwise return a list of
imports.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge branch 'master' into pr/125