Introduce application tracing profiler

[maleadt]

The current Metal.@profile is more of a GPU kernel debugger/optimizer, capturing frame captures and requiring Xcode to view the generated traces. For high-level tracing, Instruments is a better application, so this PR renames the current functionality to Metal.@capture, and introduces a new Metal.@profile that runs under xctrace (relying on some bleeding-edge ObjectiveC.jl CoreFoundation wrappers to properly detect when the tracing has started).

[maleadt]

@staticfloat: running under xctrace doesn't seem to work on CI:

(kernel) Failed to query kext info (MAC policy error 0x1).
Failed to read loaded kext info from kernel - (libkern/kext) internal error.
Instrument with name 'GPU' cannot be found

Does this ring a bell? Does the sandbox prevent kext access? I guess it's best to leave that disabled, but I was wondering.

[staticfloat]

Does the sandbox prevent kext access?

It runs inside of a Seatbelt configuration, which limits many things that it can do. We can likely figure out which kernel features are being denied and add those in, as long as they don't pose a security risk; read-only access to what kexts are loaded should be fine, but loading arbitrary kext's.... probably not a good idea.