use Exporter::Tiny

Here's a port of the Sub::Exporter stuff to use Exporter::Tiny.

Advantages of Exporter::Tiny:

* fewer dependencies
* smaller memory footprint
* makes the implementation simpler (I was able to cut out about 40 lines of code doing this)

Advantages of Sub::Exporter:

* it's been around a long time, so most of the bugs have been shaken out

I'm not massively bothered whether you adopt this change or not. Just thought I'd offer it. Test suite passes, etc, etc.