Add type stubs

[willwill2will54]

Adds type information to aid Mypy, Pyright and other type checkers. (Addresses #191)

I chose .pyi interface files so as not to interfere with the considerable effort I observed making the codebase compatible with early python versions.

I unfortunately omitted using typing for decorator use when uninitialised, as mypy is erroneously opinionated about overriding new with different return types. As a result, the following example case still raises a type error:

@div
def greeting(name):
    p('Hello %s' % name)
print(greeting('Bob'))

Happy to make modifications to the implementation if requested!

[Knio]

How can this be tested/maintained?