* implement enough of ret_err_value to pass wasm tests
 * only do the proper `@panic` implementation for the backends which
   support it, which is currently only the C backend. The other backends
   will see `@breakpoint(); unreachable;` same as before.
   - I plan to do AIR memory layout reworking as a prerequisite to
     fixing other backends, because that will help me put all the
     constants up front, which will allow the codegen to lower to memory
     without jumps.
 * `@panic` is implemented using anon decls for the message. Makes it
   easier on the backends. Might want to look into re-using decls for
   this in the future.
 * implement DWARF .debug_info for pointer-like optionals.