DON'T fall back to bare strings without good reason

[2colours]

Currently, variable lookup is done by the get method od the Stash class mainly.

For some reason, the default behavior is to take everything as bare strings, should they be missing. This can easily lead to undesirable behavior, and it should rather be the exception than the main rule.

Kind of a touchy topic, needs proper testing for the migration to an opt-in logic.