Better way to list a bunch of name-keyed signatures

[thorwhalen]

Currently, the signatures module uses a dict to express the mapping between builtin names and signatures, and uses the signature(lambda ...) way of getting signature values for these.

{
    ...
    'filter': signature(lambda function, iterable: ...),
    'map': signature(lambda func, *iterables: ...),
    'print': signature(lambda *value, sep=' ', end='\n', file=sys.stdout, flush=False: ...),
    ...
}

This has several disadvantages:

• writing signature(lambda...) is combersome
• We can't add annotation information with the lambda syntax

Find a better way to do so.

Related: issue 30