core(perf): speed up getNodePath

[paulirish]

this makes it ~20% faster.

to benchmark...

1. go to https://en.wikipedia.org/w/index.php?title=List_of_2017_albums&oldid=881381189
2. evaluate the getNodePath function
3. evaluate this:

els = Array.from(document.querySelectorAll('*')); 

console.time('hi'); 
els.forEach(getNodePath); 
console.timeEnd('hi');

[patrickhulce]

LGTM!

For others, https://developer.mozilla.org/en-US/docs/Web/API/Node/nodeValue was helpful in validating myself on what nodeValue invariants there are :)