Introducing a Stasher which is a convenience helper for interacting with
typed values for a single stash key. The stasher itself is generic and
handles casting retrieved values to the desired type.

There are three different ways to handle the value not existing or being
of the wrong type:

- RetrieveOrDie panics if a value of the desired type is not found
- RetrieveOrError errors if a value of the desired type is not found
- RetrieveOrEmpty falls back to the empty value for the type

Signed-off-by: Scott Andrews <scott@andrews.me>

Co-authored-by: Max Brauer <mamachanko@users.noreply.github.com>
Signed-off-by: Scott Andrews <scott@andrews.me>