Make `Symbol.builtin` value a function

[jasnell]

Hopefully the updates we discussed :-)

/cc @ljharb @bengl

[jasnell]

What would be extra useful is if every JS Symbol.builtin function was the same identical function, i.e., via === since it's just exposing the value of the internal slot.

Just so I'm clear... Even across realms?

const builtInFn1 = Date[Symbol.builtin];
const builtInFn2 = vm.runInNewContext('Uint8Array[Symbol.builtin]');

const yes = builtInFn1 === builtInFn2;   // true

[ljharb]

Oh, no not across realms - that'd be a communication channel. Just within the same realm (the way Array.prototype.values === Array.prototypr[Symbol.iterator])

[jasnell]

@ljharb ... updated! please take a look.

[jasnell]

Landed!