Cache the Symbol.hasInstance value

[jasongin]

For the V8 implementation of napi_instanceof, we look up the value of Symbol.hasInstance, so that we can check if a particular constructor has customized the instanceof behavior.

As an optimization, the Symbol.hasInstance value could be cached (for each napi_env), which will save some extra work of looking it up for each call to napi_instanceof.

[jasongin]

This was suggested in the node/master PR comment: nodejs/node#11975 (comment)

[gabrielschulhof]

I can take this once #208 lands.

[mhdawson]

Closing as this has landed in master.