User-visible changes to the original Douglas Laing mix package:

* New terminal mode (mixsim -t) allows to debug MIXAL programs
* New option -r to mixal allows to produce 'raw' MIX programs. It is useful to
experiment with loader routines.
* Mixal can produce a cross-reference listing (mixal -x).
* Mixal prints a warning if an ALF constant is preceeded by more than two 
spaces and followed by a printable character.
