Implement a set of "Simplified APIs" for ECM generation

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217601738

example/BUILD

@@ -16,12 +16,15 @@ package(
 filegroup(
     name = "binary_release_files",
     srcs = [
-        "simulcrypt_client.cc",
-        "test_simulcrypt_messages.h",
         ":simulcrypt_client",
     ],
 )
 
+cc_library(
+    name = "constants",
+    hdrs = ["constants.h"],
+)
+
 cc_binary(
     name = "simulcrypt_client",
     srcs = ["simulcrypt_client.cc"],