Add a BDD interface example

This adds a simple example, which shall be used for testing the new
revision of the library. It also adds a configuration to compile the
files in `examples/`
Remove unnecessary std.functional import

Not only this is useless, but I think `std.functional.partial` is
deprecated, so it breaks compilation.
Add a spawnable implementation of a reporter

It's a really simple text-only reporter, but it's what's currently
working on the multi-threaded architecture.
Add the BDD interface

Because test suite logic is now at `core`, this is nothing fancy,
really. That's something that makes me pretty happy :) It should be
trivial to add different interfaces.
Add initial working version of core

This adds the first working version of a multi-threaded `bed.core`.
There are still a bunch of things that need fixing and rethinking, but
I'm happy it works.
Tweak the .gitignore and add a `package` module

The `package` module is just for really exporting everything from bed to
the outside world.