New io

[alexkazik]

I couldn't sit still after thinking about it, here is a first draft for a Read/Write which is always exact, and the "up to" version is only used to read/write files (and DirRawStream internally).

All tests run but it's untested otherwise.

It's just a suggestion.

I can squash/remove commits/MR if unwanted.

Edit: I forgot the latest changes and force pushed it.