BF-60: JS function corrupts after round-trip back from a Python function

[Xmader]

Before the patch

def ident(x):
  print(x()) # 123.0
  return x
a = pm.eval("""(py_fn) => {
  const fn = py_fn(()=>{ return 123 })
  fn instanceof Function // true
  fn() // ??? TypeError: fn is not a function
  return fn
}""")(ident)
print(a()) #segfault

After patch

---

This pull request resolves the following internal ticket:

• https://distributive.atlassian.net/browse/BF2-60

[Xmader]

GC subsystem overhaul won't land anytime soon before our first public release.

[zollqir]

LGTM!