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>