Import iterator_adaptors from google3 (firebase#718)

* Import iterator_adapters from google3

* Remove -Wconversion which is annoyingly hard to satisfy

* Strip dependency on absl_container from iterator_adapters_test

* Format and lint iterator_adaptors

* More flexible copyright checking in Travis

.travis.yml

@@ -28,7 +28,7 @@ before_install:
 
 script:
   - "! git grep -I ' $'" # Fail on trailing whitespace in non-binary files
-  - "! git grep -L --name-only 'Copyright [0-9]* Google' | grep -v third_party | egrep '\\.(m|h|cc|mm|c)$'"
+  - "! git grep -EL --name-only 'Copyright [0-9]{4}.*Google' | grep -v third_party | egrep '\\.(m|h|cc|mm|c)$'"
   - ./scripts/style.sh test-only # Validate clang-format compliance
   - |
     if [ $SKIP_FIREBASE != 1 ]; then

Firestore/core/src/firebase/firestore/util/CMakeLists.txt

@@ -113,6 +113,7 @@ cc_library(
     comparison.h
     config.h
     firebase_assert.h
+    iterator_adaptors.h
     log.h
     ordered_code.cc
     ordered_code.h