Try to get `on.exit()` and `return()` working

[hadley]

test_that("on.exit is evaluated at end of code", {
  ev <- evaluate::evaluate(c(
    "on.exit(print('bye'))",
    "print('hi')"
  ))
  expect_output_types(ev, c("source", "source", "text", "text"))
})

test_that("return causes an early return", {
  ev <- evaluate::evaluate(c(
    "1 + 1",
    "return()",
    "2 + 2"
  ))
  expect_output_types(ev, c("source", "text", "source"))
})