Add FrameIterator to SourceMod (#716).

* Create FrameIterator type
This commit adds the FrameIterator type to core sm along with a few
methods around getting the information for each frame.

* Fix incorrect documentation
[skip ci]
* Implement KyleS's Changes
* A nit

core/logic/AMBuilder

@@ -82,6 +82,7 @@ binary.sources += [
   'CDataPack.cpp',
   'frame_tasks.cpp',
   'smn_halflife.cpp',
+  'FrameIterator.cpp',
 ]
 if builder.target.platform == 'windows':
   binary.sources += ['thread/WinThreads.cpp']