Fix bug where domains aren't re-entered when handlers are executed

[jcheng5]

Fixes tidyverse/ellmer#163

Intro to promise domains: https://gist.github.com/jcheng5/b1c87bb416f6153643cd0470ac756231

This example has always worked correctly:

library(promises)

dom <- new_promise_domain()
with_promise_domain(dom, {
  promise_resolve(TRUE) %...>% {
    stopifnot(!is.null(promises:::current_promise_domain()))
    message("success")
  }
})

Meaning, inside of a promise fulfilled handler, the promise domain is correctly in place. However, when promise handlers are nested:

library(promises)

dom <- new_promise_domain()
with_promise_domain(dom, {
  promise_resolve(TRUE) %...>% {
    promise_resolve(TRUE) %...>% {
      stopifnot(identical(promises:::current_promise_domain(), dom))
      message("success")
    }
  }
})

then this test would fail. (Side comment: currently these are both failing with the CRAN version and both passing with the PR version, something must be wrong with the way I wrote these examples!?)

Update: Sorry, I see what's wrong with the above tests--it makes sense that they both fail with CRAN. If dom was a real domain that had some kind of side effect (like setting an env var), that side effect would be in place for the first example, but the domain wouldn't be installed as the current promise domain--that was the entire point of the issue that this PR fixes. Let me try again:

library(promises)
library(withr)

envvar_domain <- function(new) {
  promises::new_promise_domain(
    wrapOnFulfilled = function(onFulfilled) {
      function(value, visible) {
        with_envvar(new, onFulfilled(value, visible))
      }
    },
    wrapOnRejected = function(onRejected) {
      function(reason) {
        with_envvar(new, onRejected(reason))
      }
    },
    wrapSync = function(expr) {
      with_envvar(new, expr)
    }
  )
}

# Passes with CRAN
with_promise_domain(envvar_domain(list(FOO="1")), {
  promise_resolve(TRUE) %...>% {
    stopifnot(Sys.getenv("FOO") == "1")
    message("success")
  }
})

# Fails with CRAN, passes with PR
with_promise_domain(envvar_domain(list(FOO="1")), {
  promise_resolve(TRUE) %...>% {
    promise_resolve(TRUE) %...>% {
      stopifnot(Sys.getenv("FOO") == "1")
      message("success")
    }
  }
})

[jcheng5]

WARNING: Some of these async tests can fail and not fail testthat, resulting in blue checks! Example