drop flexattr namespaces

Namespaces were a worthy idea, but they added a lot of complexity to both the
library code itself and every client of the flexattrs interfaces. Getting rid
of them, and having one flat namespace of both traditional fields and
flexattrs, has one huge benefit: we can "promote" flexattrs to real attributes
(and vice versa) without code changes in every client.

This frees us to have a somewhat less efficient implementation of flexattrs
because we have a smooth upgrade path for making attributes more efficient via
promotion.