Add ITS 138 source code and assemble it with MIDAS 73.

[larsbrinkhoff]

This is the earliest known preserved version of ITS from 1967. It ran on the AI lab PDP-6, with five teletypes and four GE consoles.

[larsbrinkhoff]

For more information, see:

• https://github.com/PDP-6/ITS-138
• https://gunkies.org/wiki/ITS_138