Implement backquotes in the Bel globals

[masak]

And, if at all possible, decommission the Perl backquote expander — or, better yet, relegate it to the test suite where it can serve as a "second implementation" and double-check the Bel version.

[masak]

Closing; fixed as of #277.

We didn't manage to completely get rid of the Perl backquote expander; maybe it's possible, but likely it might also make things simply too slow at this point. So it stays for now.