Replace .get-or-panic with a type-safe flow-typed version .get-strong

[andrew-johnson-4]

Maybe has a .get-or-panic accessor that gets the content if the content exists. It would also be possible to create a type-safe version that does not need to check for the existence of the missing content. This type-safe version would just be a simple field accessor so it is potentially much faster.

Name options for the new method

.get-force
.get-strong