src: stop using v8::Isolate::GetCurrent()

v8::Isolate::GetCurrent() is on its way out in upstream V8. Let's be
proactive and stop using it now.

It's up for debate what to do with NODE_SET_PROTOTYPE_METHOD(): it
operates on v8::FunctionTemplate instances and those don't provide
a means to get the owning isolate.

/cc @indutny @tjfontaine @trevnorris - think of this PR as a discussion starter more than anything else.