use Devel::LexAlias

This is a second attempt at a fix for #2 using Devel::LexAlias.

It has a fallback to using package variables when Devel::LexAlias is not available, because I think it's nice to have a pure Perl implementation. The two code paths only diverge in a couple of very small places, so I don't think it should have much impact on Eval::Closure's maintainability.