Added TODO: Document the header and turn into a single-header library. Useful for C interop with incomplete types, and also for type-erasure (PIMPL-like usage with complete C++ types as a makeshift std::any that holds only 1 pointer and 1 type could come up.)

Added TODO: Move impl-only constructs in the header into a (possible access-restricted or private) impl namespace.

Fortune for opaque_handle's current commit: Future blessing − 末吉

include/opaque.hh

@@ -1,3 +1,5 @@
+//! TODO: Document this header, then turn it into a single-header library. It's useful. We can make `src/main.cpp` our test program.
+//! TODO: Put impl-related stuff into a private namespace maybe? or at least a [[gnu::visibility("internal"/"hidden")]] one? Or perhaps their polymorphism is such that the impl namespace shouldn't be private idk... At most, internal, I'd guess. but it'd need testing.
 #pragma once
 
 #include <utility>