In certain conditions, inspecting a Proxy object can lead to a
max call stack error. Avoid that by detecting the Proxy object
and outputting information about the Proxy object itself.

Also adds util.isProxy()

Fixes: nodejs#6464

Just eliminated the isProxy call entirely, just try to get the
proxy details and if it returns undefined, we know. Still have
to cache the results either way to avoid duplicate lookup later.
Simplified the native side to return an array as suggested.
Expanded the test cases a bit more.

It's still dog slow so the showProxy option has to be enabled
for it to walk the path.

Also, since apparently functions can also be proxied, check for
that too.