util: add optional thisArg argument to promisify

[seishun]

Fixes #13338.

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)

util

[bnoordhuis]

Seems kind of pointless, to be honest. f.bind(obj) does the same thing and with similar performance.

[seishun]

Seems kind of pointless, to be honest. f.bind(obj) does the same thing and with similar performance.

I tend to agree. Let's see what others think.

[vsemozhetbyt]

Refs: http://2ality.com/2017/05/util-promisify.html#comment-3337071350

[addaleax]

This should at least also take care of the case that orig[kCustomPromisifiedSymbol] is set (in that case, it’s probably just best to return fn.bind(thisArg)).

[seishun]

I guess we don't want this.

[sclausen]

I ran into the same issue @vsemozhetbyt referenced. The errors will occur in the library to promisify and are misleading.
At least a hint in the docs would've been great.