add Worker::FileMarshaller to automatically serialise and deserialise `file: true` properties in operations.
work on FileMarshaller which uses a Contract object to find out what is a file.
adding support for contract-configured deserialising in Worker.
test that the ::run block yields the op. fixes #5.
so much easier: the file marshal representer is now created once and uses #apply.
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Merge branch 'file-marshal'

Conflicts:
	test/operation_test.rb