* Add the gint_switch() function which executes user-provided code from
  the system (CASIOWIN) context.
* Added interrupt masks to the core context (should have been there long
  ago).
* Added the gint_osmenu() function that switches out of gint to invoke
  GetKeyWait() and inject KEY_CTRL_MENU to trigger the main menu. This
  uses many CASIOWIN syscalls, but we don't care because gint is unloaded.
  Trickery is used to catch the key following the return in the add-in
  and/or display a new application frame before GetKeyWait() even finishes
  after coming back. This is only available on fx9860g for now.
* Removed any public syscall definition to clear up interfaces.
* Patched the DMA interruption problem in a weird way on fxcg50, a
  driver function will be used to do that properly eventually.
* Changed the driver model to save driver contexts in preallocated
  spaces instead of on the stack for overall less risk.
* Enabled return-to-menu with the MENU key on fx9860g in getkey().
* Changed the keyboard driver to emit releases before presses, as a
  return-to-menu acts as a press+release of different keys in a single
  driver frame, which confuses getkey().
* Fixed a really stupid bug in memcpy() that made the function really
  not work.

Improvements in the timer driver:

* Expose ETMU modules as SH7705_TMU and SH7305_TMU in <gint/mpu/tmu.h>.
* Remove the timer_t structures, using SH*_ETMU and SH*_TMU instead.
  Only interrupt gate entries are left hardcoded.
* Discovered that not only every write to the TCNT or TCR of an ETMU
  takes about 1/32k of a second (hinting at registers being powered by
  the same clock as the timer), but every write occuring while a previous
  write is pending is *lost*. This led to terrible bugs when switching
  ETMU contexts too fast in gint_switch().
* Removed an internal timer_address() function.
* Overall simplified the handling of timers and the initialization step.