skip: deprecate skip.If and replace it with assert.SkipIf

[dnephin]

Originally we thought there might be more stuff to include in the skip package. It has been a few years, and we haven't added anything.

This PR deprecates the skip package, and moves the one function to assert.SkipIf. Instead of a very small page it should be fine to move this one exported function to the assert package.

Also improves the godoc a bit, and documents msgAndArgs on assert.Assert as well.

[dnephin]

@vdemeester what do you think? Any concerns with removing the skip package and moving the one function into assert ?

[dnephin]

Fair point! My guess is that no one would really use the skip package on its own, because it doesn't do all that much. That's one of my main motivations for moving it into assert. There's no rush to merge this PR. I'll keep it open for a while so we can think about it.