Code refactor, fixed bugs and add Win64 func

[GermanAizek]

@richgel999, @cwoffenden review my changes. Thx.

[cwoffenden]

@WiredGermanAizek this isn't my project. I would suggest though that since you have a lot of changes, reviewing would be easier if they were broken into individual commits, and with a description as to why you'd make that change.

Looking at your code, there's some C++ modernisation (use of default, move semantics, for example) which could offer a better codegen, but it's only worthwhile (IMO) if it brings an improvement.

As for the Win64 additions, doesn't it only make sense for larger files?

And what are the bugs addressed?

[richgel999]

Could you describe the reasoning behind these changes? I'm pretty conservative about taking PR's. The more focused they are, the better. Thanks!