1.0.16.40: implement %SET-SYMBOL-VALUE-IN-THREAD

1.0.16.40: implement %SET-SYMBOL-VALUE-IN-THREAD

 * Eventually we may want to export this, but let's keep it internal
   for now...

 * Rename SB-THREAD::SYMBOL-VALUE-IN-THREAD to
   %SYMBOL-VALUE-IN-THREAD, and make it work with thread objects
   instead of SAPs. Also, never return the global value, but instead
   signal an error if the symbol is unbound in the thread.

 * Similarly, rename THREAD-SAP-FOR-ID to %THREAD-SAP, and make it
   work with thread objects instead of os-thread pointer values (née
   thread ids).

 * Rename CURRENT-THREAD-SAP-ID to CURRENT-THREAD-OS-THREAD.