Implement domEach utility

Optimizes internal iteration for both memory and speed by avoiding creation of Cheerio instances when operating directly on DOM elements.
Implement getAttr local helper
Avoid cheerio creation in add/remove/toggleClass
Use domEach in data accessor
Inline children lookup in find
Use domEach for traversal
Optimize identity cases for first/last/eq
Optimize siblings cheerio instance creation
Optimize filter Cheerio intermediate creation