Int64 & some questions

Hi, 
for one of my projects I'm in need of both Int64 and Int128 implementations. What I'm looking for is classes supporting all the operations that classical Int has (and easily drop them in - thing that haxe.Int64 makes not possible currently even using static extensions). 

Your Int128 seem to go that way and looks quite promising (although still lacking some things, as you pointed out in comments).

I've played a bit with your abstract and made some additions to a Int64 class (copied from Int128). I cheated by using haxe.Int64 in some places, but having a working toString() really sped up my tests.

This is really not meant as a PR, I'm just sending it for the purpose of discussing some things.
A few:

 1. Does inlining the @:ops give better performance?
 2. What you'd think about changing the @:ops into static functions and get rid of noCompletion/using?
 3. Do you have any tests/benchmark code?
 4. Are you planning to add an Int64?

I'm going to experiment more with your classes, and hopefully contribute in some way.