Implement HIR support.

The HIR instruction are officially undocumented but are well known and
described and used in many programs. The HIR instructions are just dealing
with the 8 stack internal registers.

The operations are:

   HIR CN : 82 CN

Where N is in range 0 .. 7 and correspond to the 8 internal registers with
0 being the currently displayed value (X).

And C is:
  0  : STO : X         -> Stack (N)
  1  : RCL : Stack (N) -> X
  3  : ADD : Stack (N) += X
  4  : MUL : Stack (N) *= X
  5  : SUB : Stack (N) -= X
  6  : DIV : Stack (N) /= X

This patch plus the previous (printer fix) enable me to play an old game in the emulator.

Let me know if you want me to add a documentation into the help for this.