trigger mapper compilation on class attribute access

The 0.4 properties-as-columns/comparables feature allows new idioms that may be used before mappers are compiled:

mapper(MyClass, table1)

# prepare some expressions and selects...
criteria_1 = MyClass.kind.in_('a', 'b', 'c')
sel_1 = select([MyClass.id](MyClass.id), criteria_1)

Mappers could perhaps install trigger stubs pre-compilation, something like:

class MapperPropertyStub(object):
    def __init__(self, key):
        self.key = key
    def __get__(self, obj, cls):
        assert not obj
        compile_mappers()
        return getattr(cls, self.key)

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