Allow disabling type safety checks for mutable HashMapContexts

[FruitieX]

This PR makes it possible to disable the type safety check in HashMapContext::set_value().

I tried to stay consistent with the naming of without_builtin_functions and associated functions.

I'm creating a program that's responsible for my home-automation systems, and using evalexpr I can make some simple scriptable rules and actions for it. This worked fine until I ran into a problem where I cannot assign another value to a variable if it already contains an empty value.