buffer: support boxed strings and toPrimitive

Add support for Buffer.from(new String('...')) and Buffer.from({[Symbol.toPrimitive]() { return '...'; }})

const buf1 = Buffer.from(new String('this is a test'));

class Foo {
  [Symbol.toPrimitive]() {
    return 'this is a test';
  }
}
const buf2 = Buffer.from(new Foo());

/cc @bnoordhuis @addaleax

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

buffer