The codegen operation prints an AST generated from a schedule,
given either as a schedule tree or a (union) map.
As a convenience, applying codegen to a set would result
in codegen being applied to the identity mapping on the set.
This seems to be causing confusion about the nature
of the codegen operation.  Drop the magic such that users
are aware that a schedule is needed to perform AST generation.
They can still explicitly call the "identity" operation on a set
if they really want the old behavior.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>