buffer: add buffer.transcode

Add buffer.transcode(source, from, to) method. Primarily uses ICU
to transcode a buffer's content from one of Node.js' supported
encodings to another.

Originally part of a proposal to add a new unicode module. Decided
to refactor the approach towrds individual PRs without a new module.

Refs: https://github.com/nodejs/node/pull/8075
PR-URL: https://github.com/nodejs/node/pull/9038
Reviewed-By: Anna Henningsen <anna@addaleax.net>

tools/icu/icu-generic.gyp

@@ -20,9 +20,7 @@
       'type': 'none',
       'toolsets': [ 'target' ],
       'direct_dependent_settings': {
-        'defines': [
-          'UCONFIG_NO_CONVERSION=1',
-        ]
+        'defines': []
       },
     },
     {