Add r_apply function

[oubiwann]

r_apply will take a list of results and apply them to a function:

results:r_apply(some_func, [R1, R2, ...]).

(It should goe without saying that some_func needs to have not only the appropriate arity, but also accept the #result record as the type for each argument.)

Not convinced of the usefulness of this function, so will wait on implementation.