Bump rust-argon2 dependency to 0.7

See merge request !33

Switch from rand_os crate to getrandom

See merge request !32

This extracts part of !30 into it's own PR. getrandom has a smaller dependency footprint, and the version of rand_os being used was quite out of date.

v3.0

See merge request redox-os/users!26

I think we're ready to merge, I'm happy with the state of the code and
the state of the docs. I'd like to majorly refactor before a 1.0.0
release, but this is just fine for now.

Replace rand 0.5 with rand_os 0.1

See merge request redox-os/users!27

redox_users only uses the OsRng struct which has been moved to the
rand_os crate.

Basically takes advantage of the configuration struct.
The tests still fail and I still don't know why.

For some reason that I cannot ascertain, two tests still fail. I have
examined values basically everywhere and cannot figure it out. User and
Group get Debug now though.

The tests still don't pass, but this is a big step. Solves #23.

AllUsers and AllGroups instances now take a configuration struct which
allows a little finer control over the behavior of the library. Docs
included with this commit.

Update rand crate to 0.5

See merge request redox-os/users!24

Use stages in CI

See merge request redox-os/users!23

Remove FromIter; Bump Version

See merge request !22

Gitlab CI

See merge request !21

Mggmuggins/shadowfile

This removes fs2 and linux file locking and simply runs the tests
without concurrency rather than actually fixing file locking to make
them work like they should.The value of working file locking on Linux is
debatable anyway.

I'll open an issue to track file locking with this repo, and maybe go
and do some work to get it implemented for Redox.

I used `cfg(not(target_os = "redox"))` for all the non-redox locking
behavior (including fs2), even though we only use it for tests. I'm
hoping that is alright.