move "scheme" collection to a "scheme-lib" package

Also, move remaining "srfi" libraries to "srfi-lite-lib".

In principle, "base" should depend on "scheme-lib" and
"srfi-lite-lib", and a new "base2" package would represent the new,
smaller base. But I don't think the window has yet closed on
determining the initial "base" package.

The "srfi" libraries moved to "srfi-lite-lib", instead of "srfi-lib",
to avoid creating many extra dependencies on "srfi-lib" and all of its
dependencies. The SRFIs in "srfi-lite-lib" depend only on "base",
and they are used relatively widely.
auto-fix dependencies for "scheme-lib" and "srfi-lite-lib"
move "ffi/examples" from base to "racket-doc"
raco pkg: add a missing `close-input-port' on network connection
ffi/unsafe: unbreak `_list-struct`

Broken by 0b1f96ab3d (specifically one of my changes to
Tobias's patch).
add "collects/realm/README.txt"

Guide readers of _Realm of Racket_ to the new location of
the "realm" directory.