`Choose` node type

[masonproffitt]

Described briefly in iris-hep/func_adl_uproot#46. In query languages, it's easy to form cartesian products of sets, but it seems generally fairly painful to form combinations (all distinct subsets of a particular size). I propose to add a Choose node type that does this latter operation. This is a commonly needed feature in HEP analyses. The format in qastle would be (Choose <source> <n>), where source is the original sequence and n is the number of elements in each combination. Elements are not repeated within each combination.