add exercise circular-buffer

[ekse]

The tests were ported from the Ruby version, the example implementation is similar to the Go example.

[petertseng]

Maybe could add a test for using overwrite in a non-full buffer too (like in Ruby) - create with size 2, write 1, overwrite 2, read should return 1 instead of 2

[ekse]

good point, I will add this test tonight.

[ekse]

I added the test. I actually had a bug in there due to a typo, oopsies.

[EduardoBautista]

We don't usually include the lib.rs file. Must have gotten accepted by accident in the "forth" exercise.

[ekse]

I can remove it, the reason I included was to provide the Error enum definition.

[EduardoBautista]

@ekse I don't think it's too unreasonable to expect the Enum to be defined as a part of the exercise. If you can remove it, please do.

[ekse]

I removed the lib.rs, I also merged the commits together.

[EduardoBautista]

Looks good to me! Thanks!