Add c_profile rake tasks for MacOS, using the Xcode instruments command

[dylanahsmith]

Problem

As we move more code into C, we will have less visibility into where time is being spent from the ruby profile.

Solution

Add c_profile rake tasks that uses the instruments Xcode command. I added separate tasks to this c_profile namespace for compile, render and run (compile and render). These leverage the same ThemeRunner that the profile:run task uses, which is defined in the liquid gem.