Embed test data files into test executables so that file I/O isn't
necessary during tests. This allows the tests to run on platforms that
don't have file I/O. It also makes it easier to run the tests on a
separate (virtual) machine from the build machine since the test
automation no longer needs to keep track of the test files.
Switch to Rust 2018 Edition. Fix up some build breakage for different
configurations that were found in the process of testing this,
particularly `--no-default-features`.
