buffer: make FastBuffer safe to construct

Using an explicit constructor is necessary to avoid relying on
`Array.prototype[Symbol.iterator]` and `%ArrayIteratorPrototype%.next`,
which can be mutated by users.

PR-URL: https://github.com/nodejs/node/pull/36587
Refs: https://github.com/nodejs/node/pull/36428
Refs: https://github.com/nodejs/node/pull/36532


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>