tools: add buffer-constructor eslint rule

Now that the Buffer.alloc, allocUnsafe, and from methods have landed,
add a linting rule that requires their use within lib. Tests and
benchmarks are explicitly excluded by the rule.

PR-URL: https://github.com/nodejs/node/pull/5740


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>