target triples are fragile and strings stink in general, so fix that

[stephen-hawley]

target triples are represented as a string. This is ill advised as they change. Instead, we should have a data structure that includes:

architecture (enum)
platform (enum)
operating system (enum)
os version (Version)
simulator (bool)

and a handy ToString override to generate the triple. This will make certain tasks that we do frequently much easier.

Notes to future Steve, make this an immutable struct and add methods to generate a simulator/non-simulator version and one with a changed OS.