Split C++ Func/Table/Memory/Global to their own headers (#10688)

I did my best to separate out headers but I don't understand enough C++
to fully sever `val.hh` and `func.hh` so they sort of circularly
reference each other now.

Author: alexcrichton

crates/c-api/include/wasmtime.hh

@@ -121,4 +121,3 @@ private:
 } // namespace wasmtime
 
 #endif // WASMTIME_ERROR_HH
-

crates/c-api/include/wasmtime/error.hh

@@ -0,0 +1,24 @@
+/**
+ * \file wasmtime/extern.hh
+ */
+
+#ifndef WASMTIME_EXTERN_HH
+#define WASMTIME_EXTERN_HH
+
+#include <wasmtime/extern.h>
+#include <variant>
+
+namespace wasmtime {
+
+class Global;
+class Func;
+class Memory;
+class Table;
+
+/// \typedef Extern
+/// \brief Representation of an external WebAssembly item
+typedef std::variant<Func, Global, Memory, Table> Extern;
+
+} // namespace wasmtime
+
+#endif // WASMTIME_EXTERN_HH