More invariants are enforced in the type, and less state needs to be
stored in the main sink itself. The method here is roughly that known as
"session types".
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
The new `MemorySourceAccessor` rather than being a slightly lossy flat
map is a complete in-memory model of file system objects.
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
