Enhance filters to search for no-particular-object and non-empty-collection

[idanarye]

from easypy.tokens import ANY

some_collection.filtered(foo=ANY)

should be equivalent to:

def pred(obj):
    try:
        return bool(obj.foo)
    except LookupError:
        return False

So:

• If foo is a single object, some_collection.filtered(foo=ANY) should return all objects that have a foo (which is not None, and where obj.foo doesn't throw)
• If foo is a collection, some_collection.filtered(foo=ANY) should return all objects that have a non-empty foo.

Open Question

Do we need a NONE token, which is the reverse of ANY? We can usually do some_collection.filtered(foo=[]) or some_collection.filtered(foo=None), depending on the situation, but some_collection.filtered(foo=None) will support cases where obj.foo throws, and will also be a more uniform way to do this search.