Generalize FlatReader to multiple levels

[thorwhalen]

See the FlatReader KvReader.
The user probably expects that the flattening happens at all levels, though currently it only happens on two levels.

Need a general tool for flattening views.
What we're doing here is giving access to a nested/tree structure through a key-value
view where keys specify tree paths.
Should handle situations where number layers are not fixed in advanced, but determined by some rules executed dynamically.

Related DirStore and kv_walk.

See Permute levels of nested mappings for an example need.