229 delayed_choices as single function

[chlebowa]

Closes #229

CHANGES

Replaced all_choices with delayed_choices which allows to perform delayed selection of the first or the last element of choices, as well as all elements, maintaining all_choices functionality.

IMPLEMENTATION

delayed_choices returns a function that will add the appropriate subsetting operation to the function found in the $subset element of its delayed_data argument. Atomic arguments are returned as is.
The function has a class for easy identification.

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

Selecting all choices (delayed_choices("all")) is still a special case when checking arguments in some functions (choices vs multiple) so an extra class is added to delayed_choices functions to keep track of that.

NOTE

I thought about including the possibility of selecting the n-th element but I decided agains it. If the user knows that they want the n-th element speficically, they likely know what it is and can define it literally. Also, it opens the possibility of a range of elements (n:m) and that quickly escalates beyond reasonable granularity.
Let me know if you want an n-th choice option anyway, it can readily be added with this implementation.

ALTERNATIVE

See #232 for alternative implementation with separate functions.

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[chlebowa]

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