[Pitch] Subprocess: Reporting the CPU time and peak memory of terminated child processes

This pitch proposes adding resource usage reporting to swift-subprocess: the CPU time and peak memory of every child process it runs. Subprocess has to provide this itself, because user code cannot obtain it through today's API surface. Author: @jakepetroules Implementation: swiftlang/swift-subprocess#226 Introduction ExecutionResult currently tells you how a subprocess ended, through terminationStatus. This pitch adds a ResourceUsage value with the child's user CPU time, system CPU time, and...