ascending and descending tuple get and set

[oscbyspro]

From oscbyspro/Numberick#8

In addition to High and Low initializers (#7), I think I want component accessors:

var  ascending: (low:  Low,  high: High) { get set }
var descending: (high: High, low:  Low ) { get set }

These enable otherwise unergonomic use cases. Imagine passing inout arguments:

mutate(&integer .ascending)
mutate(&integer.descending)