229 delayed_choices as separate funcitons

[chlebowa]

Closes #229
Alternative to #231

CHANGES

Added first_choice and last_choice to extend all_choices functionality to subsets of available choices.

IMPLEMENTATION

all_choices, first_choice, and last_choice are functions of class delayed_choices. delayed_choices functions will add the appropriate subsetting operation to the $subset element of their delayed_data argument. Atomic arguments are returned as is.

if (inherits(selected) "delayed_choices") {
  selected <- selected(choices)              # modify delayed choices
} else {
  selected <- choices                        # use choices as is
}

all_choices is still a special case when checking arguments in some functions (choices vs multiple) so an extra class is added to the all_choices function to keep track of that.

[llrs-roche]

I like this approach more as all_choices is already released and this approach would keep it around.

About the exact implementation and other details I haven't reviewed it, but at first glance I'd like the explore the call to Reduce on R/delayed_choices.R lines 80 and 59. Maybe it can be reviewed & merged on the early January. Thanks!