Should use R_TopLevelExec when executing callbacks

[wch]

Currently, callbacks that are executed by run_now can affect the caller, but we don't want this.

My understanding is that if we execute the callbacks with R_TopLevelExec, it acts as a barrier which exceptions and returns won't cross.

Here's an example that demostrates the return issue (adapted from callCC) where an early return affects the run_now, and prevents subsequent callbacks from being executed:

f <- function() {
  delayedAssign("throw", return(100))
  
  later::later(function() {
    throw
  }, 2)
  
  later::later(function() message("hello"), 2)
  
  later::run_now(3)
  200
}

f()
#> [1] 100

The f() returns 100, but that's not what we'd expect. Also, the subsequent callback with message("hello") just doesn't execute. For some reason, after running this code, the callbacks no longer run at an idle console. I think this is probably related to the early return. Calling run_now() again does execute the message("hello"), however.

Although the example above uses an unusual case (an early return) I believe a similar thing happens with exceptions, and I think that the R_TopLevelExec will prevent these problems.

[wch]

This was implemented as part of #84.