[DEPRECATE] program/input is now program/code

[Rob Beezer]

A #program can just be a blob of code, no structure. That was a recent
enhancement. Otherwise, to structure a #program, the main body of code went in
an #input (modeled on some early naming choices for #sage).

To allow more, and better, structure, #input is now #code. This is a cosmetic
change, and we will honor your intent indefinitely. So you will get (many?)
warnings, but there is no urgency.

Why? Andrew Scholer has made a lot of improvements for interactive programs and
this change will help make some of those new features more apparent for authors.
(Thanks, Andrew!) Announcement of these features coming next. (As usual,
please report any unintended chages on -support or -dev. Thanks.)

Rob