[metapost] Original Euler sources

[Karl Berry]

    Savannah for GNU 3DLDF:
    https://git.savannah.gnu.org/cgit/3dldf.git/tree/fonts/source/public/ams/euler

Thanks Laurence. Inspired by this, I also made them available under
https://tug.org/historic/, namely (and mirrors, in due course)

 rsync://tug.org/historic/fonts/euler/

I found two releases readily available, from 1995 and 1991. The 1995
version (the last as Metafont, I believe), was actually already in the
historic archive, under macros/ams/amsfonts*, but that was somewhat
obscure location, so maybe I made a symlink. The ams area also has tfms,
but I did not try to copy those.

Also, for the record, I feel I should mention that Hermann Zapf drew new
versions of many Euler characters in the 2000s, a project with Hans,
Taco, and Volker. Hans wrote a short article about it, showing some of
the changes: https://tug.org/TUGboat/tb29-2/tb92hagen-euler.pdf
They also added 

Those fonts are available as Type 1's, in the current amsfonts package.

Best,
Karl