Overload etag/etagger (and other potential pairs, e.g.: key/keyer)

[mikeboers]

If we restrict the types that are allowed to be used as etags (and/or user-specified keys, see #4), then we could allow either a scalar etag or a function to create said etag to be passed via the same etag kwarg.

Potential set of types could be:

• strings
• ints
• bools
• None
• tuples of these

Basically, hashable builtin types.

[mikeboers]

Also, overload the methods on a decorated function to be used as decorators themselves:

@memo
def myfunc(a, b):
    pass

@myfunc.etag
def myfunc(a, b):
    return str((a, b))

Just need a relatively comprehensive iscallable to do the detection, since we have already declared that memoized callables may only use hashable arguments.

[mikeboers]

Would be great if this would also support direct assignment:

myfunc.etag = common_etagger