Isapprox -> base

This is a "new version" of #3076, aiming to migrate the functionality for `isapprox(x::Number, y::Number)` into base.

For previous discussions about functionality, please refer to the old pull request. (New discussions go here, of course.) However, the old pull request was tainted by my lack of git-fu, so for code review please only look at the code in this issue.