Improve factorization of "unused space"

[ianfixes]

Use more sophisticated logic to calculate free space remaining

Regarding code contributed by @jgfoster in #308

          output = @backend.last_msg
          puts output
          i = output.index("leaving")
          free_space = output[i + 8..-1].to_i

We need to do the following:

1. Move the space calculation into the backend
2. Specify the units (Bytes, kB, etc) in the messaging for the CLI option
3. Use a more sophisticated regex match to extract the number value, and possibly add unit testing for that
4. Report space requirements as a separate check from the compilation itself