A test for '_' in Positron::Environment

Also testing this in Positron::Expression, for completeness.
Refactored interpolation.

The caller of _process* now is responsible for the interpolation
step. This is slightly cleaner, but not by much. Still, it works
with '<' and '-' syntax.
First work on the wrapping construct: List and Text context.

The tests are incomplete in this commit, and Hash context is still
not handled.