[Bug-apl] caller known in native function

[Juergen Sauermann]

Hi,

I have changed the NativeFunction class so that so that the
called function (eval_AB(), eval_B(), ...) in the shared library
can access the calling object (of class NativeFunction).

This can be used if the called function wants to know, for example,
the name under which is was ⎕FX'ed in APL.

Also moved UCS_string::to_string to emacs.hh and adapted the emacs_mode
files accordingly. See SVN 203.

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