Move some numeric utility code from rtc_base/ to rtc_base/numerics/

Specifically, I'm moving

  safe_compare.h
  safe_conversions.h
  safe_minmax.h

They shouldn't be part of the API, and moving them to an appropriate
subdirectory of rtc_base/ is a good way to keep track of that.

BUG=webrtc:8445

Change-Id: I458531aeb30bcf4291c4bec3bf22a2fffbf054ff
Reviewed-on: https://webrtc-review.googlesource.com/20860
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20829}
diff --git a/rtc_base/checks.h b/rtc_base/checks.h
index 9f5fc2e..e9a19e6 100644
--- a/rtc_base/checks.h
+++ b/rtc_base/checks.h
@@ -36,7 +36,7 @@
 #include <sstream>
 #include <string>
 
-#include "rtc_base/safe_compare.h"
+#include "rtc_base/numerics/safe_compare.h"
 
 // The macros here print a message to stderr and abort under various
 // conditions. All will accept additional stream messages. For example: