punycode: deprecate punycode module

Currently, the punycode module is used in exactly one spot within
core (the url parser). With the recent switch to using the much
faster ICU based punycode implementation by default, the punycode
module is now only used when node happens to be built without icu.
This change moves the punycode module into internal and hard
deprecates `require('punycode')`. The hard deprecation notice is
only printed in ICU builds.

When the new WHATWG URL implementation lands, users will have access
to the URL.domainToUnicode() and URL.domainToASCII statics that are
defined as part of the standard interface.

The next step (in the next major) is to make it so that
internal/punycode.js is only included if the Node.js binary is built
without ICU.

.eslintignore

@@ -1,5 +1,5 @@
 lib/internal/v8_prof_polyfill.js
-lib/punycode.js
+lib/internal/punycode.js
 test/addons/??_*/
 test/fixtures
 test/disabled

doc/api/punycode.md

@@ -1,6 +1,6 @@
 # punycode
 
-    Stability: 2 - Stable
+    Stability: 0 - Deprecated
 
 The `punycode` module is a bundled version of the [Punycode.js][] module. It
 can be accessed using: