n-api: cache Symbol.hasInstance

This improves the performance of napi_instanceof() by retrieving
Symbol.hasInstance from the global object once and then storing a
persistent reference to it in the env.

PR-URL: https://github.com/nodejs/node/pull/12246


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>