Wrap functions in class with decorator

[Dreamsorcerer]

The decorator should be simplified to a simple function, which should then return an object which wraps the passed function. Something along the lines of:

class Wrapper:
    def __init__(self, func):
        self.func = func

    def __call__(self, *args, **kwargs):
        # Get from cache...
        # Or call self.func(*args, **kwargs)

def cached(func):
    return Wrapper(func)

This simplifies the logic, and makes it easy to add methods on to our function (related: #538). For example, to call the function while forcing the cache to update could look something like:

@cached
def foo(...):
    ...

# Force update
foo.refresh(...)