Move headers under the hf directory.

This helps distinguish local headers from the more official headers.

Change-Id: I535c1b44081b9d60ba63666cd1ad514aaaf2c68d
diff --git a/.clang-tidy b/.clang-tidy
index f426a64..bc5bcaa 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -1,2 +1,3 @@
 Checks: 'readability-*,portability-*,performance-*,misc-*,bugprone-*,modernize-*'
 HeaderFilterRegex: '.*'
+FormatStyle: file