Add operator `INCLUDE`

[python-processing-unit]

Add operator BOOL INCLUDE(STR module).

INCLUDE acts exactly like IMPORT except the the imported names are unqualified.

For example, if module foo is:

STR foo = "foo"

Then either of these would be correct:

IMPORT("foo")
PRINT(foo.foo)

INCLUDE("foo")
PRINT(foo)

But these would be incorrect:

IMPORT("foo")
PRINT(foo)

INCLUDE("foo")
PRINT(foo.foo)

Update the specification, tests, and implementation.