Cached functions: cast arguments to hashable types before invocation

[sanjayankur31]

For the few methods where we use caching, one has to use tuples instead of lists because the arguments to these methods must be hashable:

https://docs.python.org/3/library/functools.html#functools.lru_cache

If the user uses a list instead of a tuple, they get a "must be hashable" error, which will be difficult for them to understand.
So, where possible, we should cast lists to tuples ourselves, and where not possible, add assertions to inform the user what the correct form of the function call is.