Draft: Storage: use strings instead of bytes

This is proof-of-concept to show what it takes to replace bytes by string in the storage layer.

I think this is mostly ok. There is only one place where we might copy too much: Data_encoding.Binary doesn't take string as input so we have to copy arguments. I don't see why we can't add this ;-) For the record, Irmin's own data-encoding library is using string streams as input buffers and that works nicely.