- MapperProperty gets its .key attribute assigned early, in _compile_property.

- MapperProperty gets its .key attribute assigned early, in _compile_property.
MapperProperty compilation is detected using a "_compiled" flag.
- A mapper which inherits from another, when inheriting
the columns of its inherited mapper, will use any
reassigned property names specified in that inheriting
mapper.  Previously, if "Base" had reassigned "base_id"
to the name "id", "SubBase(Base)" would still get
an attribute called "base_id".   This could be worked
around by explicitly stating the column in each
submapper as well but this is fairly unworkable
and also impossible when using declarative [ticket:1111].