DTrace support: function calls, GC activity, line execution

Tested on macOS 10.11 dtrace, Ubuntu 16.04 SystemTap, and libbcc.

Largely based by an initial patch by Jesús Cea Avión, with some
influence from Dave Malcolm's SystemTap patch and Nikhil Benesch's
unification patch.

Things deliberately left out for simplicity:
- ustack helpers, I have no way of testing them at this point since
they are Solaris-specific
- PyFrameObject * in function__entry/function__return, this is
SystemTap-specific
- SPARC support
- dynamic tracing
- sys module dtrace facility introspection

All of those might be added later.
diff --git a/Misc/ACKS b/Misc/ACKS
index 414cda9..0aeb1b1 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -120,6 +120,7 @@
 Thomas Bellman
 Alexander “Саша” Belopolsky
 Eli Bendersky
+Nikhil Benesch
 David Benjamin
 Oscar Benjamin
 Andrew Bennetts