readline: deprecate undocumented exports

This commit moves several of readline's undocumented functions
into an internal module. Specifically, isFullWidthCodePoint,
stripVTControlCharacters, getStringWidth, and emitKeys are
moved to the internal module. The existing public exports
of the first three functions are given a deprecation notice.

Refs: https://github.com/nodejs/node/pull/3847
Fixes: https://github.com/nodejs/node/issues/3836
PR-URL: https://github.com/nodejs/node/pull/3862


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>