Allow using set_working_tape as a function decorator

[JHopeCollins]

Description

This PR uses the ContextDecorator class allow set_working_tape and stop_annotating to be used as function decorators.

The no_annotations decorator is now implemented as an instance of stop_annotating. Because no_annotations contexts could be nested, we need to keep track of the original annotation states at entry to each context separately. This is done by pushing/popping from a stack in __enter__ and __exit__.

set_working_tape decorator

The two functions below are equivalent:

@set_working_tape()
def decorated_function(*args, **kwargs):
    # do something here
    return ReducedFunctional(functional, control)

def context_function(*args, **kwargs):
    with set_working_tape():
        # do something here
        return ReducedFunctional(functional, control)

stop_annotating decorator

The two functions below are equivalent:

@stop_annotating()
def decorated_function(*args, **kwargs):
    # do something here

@no_annotations
def decorated_function(*args, **kwargs):
    # do something here