This commits a bazel-based build system for C#
using `rules_dotnet`. External dependencies are managed
via `paket`, and updates to the generated bazel files
are done via `./update-deps.sh`.
We're providing our own (minimal) test runner for `xunit`
tests.
When running `dotnet paket update` or `dotnet paket install`, `paket`
forcefully reformats the project files. This is unfortunate.
One option is to accept these changes, as they're not harmful.
They do mean that each project includes the Paket restore targets individually,
instead of doing so via `Directory.Build.targets`.
Another option would be to not merge this PR, and then I'll document that
the changes to the csproj files should be ignored when running `paket` instead.
I don't really mind either way.
