Feature request: group ambiguities by function

[fingolfin]

Right now ambiguities seem to be reported in a seemingly arbitrary order (perhaps the order they are encountered in?)

I am looking at a project which has 130 ambiguities (too many sigh). To get a better overview, it would be kinda nice if the ambiguities were group by function: i.e. first all ambiguities in function aaaaa (if that existed), then those in bbbbb, etc. (just using sort on the function names -- if they are "weird" or involve kwarg or whatever, fine, sort them whatever way, but however it is it won't be worse than what we have right now, IMHO).

[lgoettgens]

I'll make a PR later today