[wpilibc] Clean up include files (#2708)

Based on run of include-what-you-use.org to identify unused include files in various .h and .cpp files.

The changes mostly fall into 3 categories:
- Actually unused includes - copy-paste errors, not removing includes after cleaning up code, etc
- A too-broad include used where a more specific (and hopefully smaller) header will do
- Interface .h files including headers only needed by the .cpp implementation - moving from .h to .cpp
  will mean that code which uses the .h doesn't pay the price of processing the header file they don't need

hal/src/main/native/sim/Extensions.cpp

@@ -12,12 +12,9 @@
 #include <wpi/Path.h>
 #include <wpi/SmallString.h>
 #include <wpi/StringRef.h>
-#include <wpi/mutex.h>
 #include <wpi/raw_ostream.h>
 #include <wpi/spinlock.h>
 
-#include "hal/HAL.h"
-
 #if defined(WIN32) || defined(_WIN32)
 #include <windows.h>
 #else