Filter: Bitfield eattrs reading / writing moved to filter code

Filter: Bitfield eattrs reading / writing moved to filter code

Before this change, fetch-update-write and bitmasking was hardcoded in
attribute access code cased by the attribute type. Several filter
instructions are used to do it instead.

As this is certainly going to be a little bit slower than before, the
switch block in attribute access code should be completely removed in
near future, helping with both performance and code cleanliness.

The user interface should have stayed intact.