Revert "Revert "Implement call combiner""

This reverts commit bf19961d0a49b43cb528392efeb4880eeebb9b5e.
diff --git a/build.yaml b/build.yaml
index a43ad6a..5264b4a 100644
--- a/build.yaml
+++ b/build.yaml
@@ -200,6 +200,7 @@
   - src/core/lib/http/format_request.c
   - src/core/lib/http/httpcli.c
   - src/core/lib/http/parser.c
+  - src/core/lib/iomgr/call_combiner.c
   - src/core/lib/iomgr/closure.c
   - src/core/lib/iomgr/combiner.c
   - src/core/lib/iomgr/endpoint.c
@@ -352,6 +353,7 @@
   - src/core/lib/http/format_request.h
   - src/core/lib/http/httpcli.h
   - src/core/lib/http/parser.h
+  - src/core/lib/iomgr/call_combiner.h
   - src/core/lib/iomgr/closure.h
   - src/core/lib/iomgr/combiner.h
   - src/core/lib/iomgr/endpoint.h