Ensure mono-generated DynamicSites don't get translated.
Performance tuning:
  Initially size JSAstVisitor stacks to 32 elements (the default behavior for Stack<> is tuned poorly for our uses)
  Make VisitorCache use thread-local data structures instead of concurrent data structures
  Make JSExpression.GetValueName much, much faster
  Make traversal array records faster by hoisting some of the per-element work into a once-per-record setup cost
  Kill some LINQ use in GetAllVariablesForMethod
New spidermonkey was incredibly slow, so bleh. Revert "Add an updated build of spidermonkey"

This reverts commit f31e642cea05e997624ccf1a35b804d1a018cd25.
Do traversal data lookup once per node in the constructor instead of doing it every time an enumerator is constructed.