Add function to easily find not covered code sections

When editing code I want to be able to check easily where the not covered code sections are. In a larger file it is tedious to scroll through the whole file watching the fringe, if there is a not covered section.

I would like to have two features:

* a function `cov-next-uncovered` to jump to the next uncovered code section after `(point)`. If there is none then it should start from the beginning of the file. If there is no uncovered section at all, there should be a message like `No uncovered section in file.`
* the mode lighter should indicate how many lines are uncovered.

I would volunteer for the implementation.