Extract Unit to top-level

Summary:
[Folly] Extract `Unit` to top-level.

It was in `folly/futures/`, but this diff moves it to `folly/`. It is needed for futures, but it is more general than futures and can be used separately.

Users must replace `folly/futures/Unit.h` with `folly/Unit.h`.

Also codemods existing usage sites:

```
hg grep -lw folly/futures/Unit.h | xargs perl -pi -e 's,\bfolly/futures/Unit\.h\b,folly/Unit.h,g'
```

Reviewed By: igorsugak

Differential Revision: D3314280

fbshipit-source-id: 16279b76b1d24529bec49077ccb36cd7d39f23b8

folly/Makefile.am

@@ -175,7 +175,6 @@ nobase_follyinclude_HEADERS = \
 	futures/Timekeeper.h \
 	futures/Try-inl.h \
 	futures/Try.h \
-	futures/Unit.h \
 	futures/detail/Core.h \
 	futures/detail/FSM.h \
 	futures/detail/Types.h \
@@ -348,6 +347,7 @@ nobase_follyinclude_HEADERS = \
 	Traits.h \
 	Unicode.h \
 	Function.h \
+	Unit.h \
 	Uri.h \
 	Uri-inl.h \
 	Varint.h \

folly/futures/Timekeeper.h

@@ -17,7 +17,7 @@
 #pragma once
 
 #include <folly/futures/detail/Types.h>
-#include <folly/futures/Unit.h>
+#include <folly/Unit.h>
 
 namespace folly {
 

folly/futures/Try.h

@@ -24,7 +24,7 @@
 #include <folly/Memory.h>
 #include <folly/Portability.h>
 #include <folly/futures/FutureException.h>
-#include <folly/futures/Unit.h>
+#include <folly/Unit.h>
 
 namespace folly {
 

folly/futures/test/FutureTest.cpp

@@ -17,7 +17,7 @@
 #include <gtest/gtest.h>
 
 #include <folly/futures/Future.h>
-#include <folly/futures/Unit.h>
+#include <folly/Unit.h>
 #include <folly/Memory.h>
 #include <folly/Executor.h>
 #include <folly/dynamic.h>

folly/test/Makefile.am

@@ -215,6 +215,9 @@ indestructible_test_SOURCES = IndestructibleTest.cpp
 indestructible_test_LDADD = libfollytestmain.la
 TESTS += indestructible_test
 
+unit_test_SOURCES = UnitTest.cpp
+unit_test_LDADD = libfollytestmain.la
+TESTS += unit_test
 
 futures_test_SOURCES = \
     ../futures/test/CollectTest.cpp \
@@ -242,7 +245,6 @@ futures_test_SOURCES = \
     ../futures/test/TimekeeperTest.cpp \
     ../futures/test/TimesTest.cpp \
     ../futures/test/TryTest.cpp \
-    ../futures/test/UnitTest.cpp \
     ../futures/test/UnwrapTest.cpp \
     ../futures/test/ViaTest.cpp \
     ../futures/test/WaitTest.cpp \

folly/futures/test/UnitTest.cpp → folly/test/UnitTest.cpp

@@ -16,7 +16,7 @@
 
 #include <gtest/gtest.h>
 
-#include <folly/futures/Unit.h>
+#include <folly/Unit.h>
 
 using namespace folly;