Detect eval context

This module is very useful and CPANTS uses it to calculate Kwalitee indicators like prereq_matches_use. As my understandings, CPANTS currently compares the result of this module with union of prereqs and optional prereqs.
If this module would detect optionality of using modules, that is use in eval context or out of eval context, we could check prereqs more precisely.

I added accessors with suffixes _in_eval / _out_of_eval, and tests for them.