[IMP] fields: add `__slots__` on all fields

Introduce slots on all field classes for common attributes; slots take much
less memory than a `__dict__`.  The other attributes are stored in a dictionary
`_attrs`; all fields with an empty value for `_attrs` (common case) share the
same empty dictionary.  This saves quite some memory (around 4.5Mb per
registry), given the number of field instances created for a registry.

Another mechanism is used for the default values of attributes, since slots
cannot be assigned on classes.

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