bug#23850: 25.0.95; Deprecate string-to/as-unibyte/multibyte

[Leo Liu]

As discussed in http://thread.gmane.org/gmane.emacs.devel/204621:

,----[ Stefan Monnier ]
| The correct way to use them is to not use them and use
| encode/decode-coding-string instead.
`----

,----[ Eli Zaretskii ]
| Don't use them, period. They are remnants from the era long gone,
| before Emacs used Unicode and UTF-8 as the basis for its internal
| representation of characters. Nowadays the only place where such
| conversions are sometimes needed are on the C level, deep within Emacs
| internals. Applications should never use them.
`----

Leo