shared: Introduce new top-level dir and move 1st party lib code there.

This commit moves all first-party code developed for this project from lib/
to shared/, so that lib/ now only contains third-party code.

The following directories are moved as-is from lib to shared:

    lib/libc            -> shared/libc
    lib/memzip          -> shared/memzip
    lib/netutils        -> shared/netutils
    lib/timeutils       -> shared/timeutils
    lib/upytesthelper   -> shared/upytesthelper

All files in lib/embed/ have been moved to shared/libc/.

lib/mp-readline has been moved to shared/readline.

lib/utils has been moved to shared/runtime, with the exception of
lib/utils/printf.c which has been moved to shared/libc/printf.c.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge