suggestion: deprecate `std/collections` APIs in favour of new `Set` methods

[iuioiua]

I suggest the following now that new Set methods are available in Deno:

1. Deprecate intersect() in favour of Set.prototype.intersection()
2. Deprecate union() in favour of Set.prototype.union()

Removals should happen in std/collections@1. However, this might be too soon as these APIs are not fully supported in other JavaScript runtimes.