Implement `make_return()`

[kinto-b]

As per #2, we want a formal way to specify return values that unifies make_*() variants.

Something like this should work:

make_return <- function(x) {
    signalCondition(rlang::cnd("makepipe_return_cnd", res = x))
    invisible(x)
}

make_with_source <- function(...) {
  ...
  out <- tryCatch(
    source(...),
    makepipe_return_cnd = function(x) {
        x$res
    }
  )
 ... 

  out
}