[int128] Some thoughts

Hi Matt,

Thanks for writing this library - it looks very useful and certainly 
fills a hole.

This is not a review, just some thoughts from a quick look at the docs 
and source code. From the docs all your use cases are relevant to us, 
and in particular the third for financial computing. We do exactly that. 
Currently we build on gcc and use `__int128` with a few workarounds. In 
general that's been fine. Replacing with `int128` would likely bring us 
little value at this stage but certainly I could see good value for any 
new projects and especially for those looking for portability. Your 
benchmark results were encouraging and might tempt me to consider using 
this in the future as a result. I would remark I see the comparison with 
the abseil 128 numbers interesting WRT addition and signed vs unsigned. 
It looks like your implementation is optimised for unsigned whereas as 
abseil favours signed? FWIW I'd see more value in optimising 
mathematical operations for signed integers. If you are using unsigned 
ints for that you probably have a design issue.

In looking over the code (briefly) I got to wondering if you could apply 
similar principles to a 256 bit type. For us we always need 128 and 256 
bit integer types (we do a lot of EVM based work which uses 256 bit 
ints). Currently we are forced to use boost.multiprecision for 256 bit 
ints which has all the issues you mention and would be a very obvious 
replacement for us. Like many who need int128 today, they already have a 
workaround, but nothing useful exists for 256 bit numbers which are very 
common in the crypto and blockchain world. Would it be challenging to 
develop a similar library for 256 bit support given your experience of 
developing this 128 library?

As I said I see value in Int128, but it is more a nice-to-have for us 
since we have something lightweight already (__int128) but would dearly 
love to see a solid Int256 library that perhaps compliments Int128. If I 
get more time later in the week I might try and do a proper review.

Jamie

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