Convert request params based on function annotations.

[coady]

With the increasing focus on type hints in Python, having a tool which automatically converted query params (with exception handling) would be a nice feature.

It's common for apps to have boilerplate code for converting request params, and returning 400-levels errors on failure.

@cherrypy.tools.params(exception=ValueError)
def resource(self, limit: int):
    assert isinstance(limit, int) # a ValueError would have already raised a 400