ODK and Shared Libraries

In this code drop we introduce the ODK dependency. The reference
implementation has been updated to make use of the ODK and the related
tests have been included.

In addition, we have included an example of how a shared libraries can
be created. This will allow make it easier to test and verify different
implementations of the API.

Most other changes introduce by this code drop were made to clean-up the
reference implementation and limit dependencies.

api/export.h

@@ -0,0 +1,12 @@
+// Copyright 2020 Google LLC. All Rights Reserved.
+
+#ifndef WHITEBOX_API_EXPORT_H_
+#define WHITEBOX_API_EXPORT_H_
+
+#if defined(_WIN32)
+#define WB_API _declspec(dllexport)
+#else
+#define WB_API __attribute__((visibility("default")))
+#endif
+
+#endif  // WHITEBOX_API_EXPORT_H_