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- more precise specification of the dependent crates

num-traits currently fails to build with i128 feature enabled, even though we have i128 support, since autocfg can't detect it.

Not sure if this is necessary.

Using the standard sys_exit when running on qemu, results in the return code being dropped and qemu always exiting with 0. This commit introduces exiting qemu via a debug device, which allows passing a return code.

Adds an initial version of integration tests. The tests are located in
the top level of the 'tests' directory. Integration tests are executed via
the hermit_test_runner.py script, which can use either uhyve or qemu to execute
the tests.
In this initial version, using the test_harness is deactivated.

see http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html
for more information

* Refactor: Fix Clippy warnings

- Added support for empty println!()
- use short hand initialization when possible
- replace if x==false with if !x
- replace unwrap_or(function_call()) with unwrap_or_else(|| function_call())
- and others

* Update to Rust edition 2018
 - Mostly just adds crate::

- requires PR rust-lang/rust#71839
- consequently, the latest nightly compiler has to be used

=> we can remove the spin crate from Cargo.toml

virtio-fs/fuse: add file opening/closing/creating/reading/writing/unlinking. Currently only 8 bytes aligned writes supported, error handling mostly missing