base: Add "fit" library

libfit is a header-only library used in Fuchsia SDK providing
immproved control flow and memory management beyond the C++17
standard library. This includes:

- Move-only function objects: fit::Function / Callback / Closure;
- Move-only RAII deferred action wrapper: fit::defer;
- Thread safety checker and thread-safety annotations.

The library is located under android::base::fit namespace, and is
modified to fit the coding style of android::base libraries.

No-Typo-Check: keep the identifiers the same as those used in the original Fuchsia repository

Change-Id: I785f4c0d22a9b2597097719432dbb0d147bd80d8
diff --git a/BUILD.gn b/BUILD.gn
index a9564fe..b5af93e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -14,6 +14,13 @@
     "android-emu/android/base/files/Stream.h",
     "android-emu/android/base/files/StreamSerializing.cpp",
     "android-emu/android/base/files/StreamSerializing.h",
+    "android-emu/android/base/fit/Defer.h",
+    "android-emu/android/base/fit/Function.h",
+    "android-emu/android/base/fit/FunctionInternal.h",
+    "android-emu/android/base/fit/Nullable.h",
+    "android-emu/android/base/fit/ThreadChecker.h",
+    "android-emu/android/base/fit/ThreadSafety.h",
+    "android-emu/android/base/fit/UtilityInternal.h",
     "android-emu/android/base/ring_buffer.c",
     "android-emu/android/base/synchronization/AndroidConditionVariable.h",
     "android-emu/android/base/synchronization/AndroidLock.h",