[RFC] Remove support for basis knits?

[Aaron Bentley]

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All knits can have a 'basis'.  This basis knit is used whenever
possible, for getting data.

However, it appears they are broken.  To wit:

            for comp_id in basis_versions:
                data_pos, data_size =
basis._index.get_data_position(comp_id)
                records.append((piece_id, data_pos, data_size))
            components.update(basis._data.read_records(records))

basis_versions is a list of all the required versions that are present
in the basis.  If it is non-empty, then (eventually)
records.append((piece_id, data_pos, data_size)) is called.  However,
piece_id is undefined.  Since our test suite passes, it appears this
code is not tested, so basis_knits are probably unused.

Supporting basis knits uglifies our code somewhat, and makes it harder
to refactor (as I'm doing for log performance).  Therefore,  I propose
we remove support, until such time as they're actually being used.

Aaron
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