As part of resolving a resource, record its MD5 hash in the output
metadata. In order to enable this, resource resolution is now done via
the context, rather than directly via the ResourceResolver (in order not
to introduce a context dependency in the Gui object, context now
emulates the resolver interface).
Set target_binary as a class, rather than instance, attribute. This
happens only only once per run, and setting it as instance attribute the
first time, makes it unavailable for subsequent instances of the same
workload.
