Products.btreefolder2

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- `keys`, `values` and `items` methods are now exactly the same as
`objectIds`, `objectValues` and `objectItems`. They did the same before
already but duplicated the code.

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- `objectValues` and `objectItems` no longer do a special handling when no
special `spec` is requested as `objectIds` already does the correct
handling.

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- Make sure that methods returning objects return them Acquisition wrapped.

- Be more careful in calling our own keys, values and items methods, as
sub-classes might have overridden some of them.

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- Changed the `objectIds`, `objectItems` and `objectValues` methods to use the
internal OOBTree methods directly if no `spec` argument is passed.

- Change implementation of `keys`, `items` and `values` method to access the
`self._tree` OOBTree methods directly. This avoids lookups in the meta_types
structures.

- Implement the full dictionary protocol including `__getitem__`,
`__delitem__`, `__setitem__`, `__nonzero__`, `__iter__` and `__contains__`.

- Released as separate package.