Around=)

The `$around` method on a stageRunner allows for wrapping a terminal node in some code.

```R
sr <- stageRunner$new(new.env(), list(a = function(e) cat('2')))
sr$around(list(a = function(e) { cat('1'); yield(); cat('3') })))
sr$run() # will print 123 instead of just 2, because the above function "wraps around" the terminal node
```