Introduce core ProtoZero support classes (Part 1)

Introduces the core classes to deal with zero-copy
zero-malloc append-only protobuf generation.
The actual ProtoZeroMessage will come in a separate
CL.

Change-Id: I4b278cfec6409f745532b17e9adaf2bb09ee2c92
diff --git a/BUILD.gn b/BUILD.gn
index 627e062..04236d6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -29,6 +29,7 @@
     "//libftrace:libftrace_unittests",
     "//libtracing:libtracing_benchmarks",
     "//libtracing:libtracing_unittests",
+    "//protozero:protozero_unittests",
     "//tools/ftrace_proto_gen:ftrace_proto_gen_unittests",
     "//tools/sanitizers_unittests",
   ]