Add useFuture() instead of options(future=...)

[HenrikBengtsson]

Add useFuture() instead of options(future=...) with the advantage of immediate validation and lookup, e.g.

useFuture(lazy)
useFuture("lazy")
useFuture(eager)
useFuture(c("lazy", "eager")[2])

But then, how to check which future is in use? Is there a better name than useFuture().

[HenrikBengtsson]

What about plan(), which works for both setting and getting, e.g.

> plan(lazy)
> plan("lazy")
> plan()
"lazy"

Also, make it easy to modify the default arguments of a future evaluator function, e.g.

plan(eager, local=FALSE)

Internally, this will do something like:

eager_tmp <- eager
formals(eager_tmp)$local <- FALSE