Add component macro and example

[maan2003]

The component macro helps to wrap the function body inside Cx::begin_loc and Cx::end to avoid key collision.

[maan2003]

The name_of_cx function is very ugly but is simplest among the alternative I have tried out.

[raphlinus]

Looks good, thanks! I haven't reviewed the proc macro logic carefully, but certainly based on the example it looks like it's doing what it should be.