Add write support

[ngirard]

Hey,

I'm involved in the IconPie project, and we are contemplating using your crate.

Unfortunately we would need to be able to write data, which is currently not implemented, unless I'm mistaken.

Could you consider adding write support to mini_fs ?

Cheers!

[germangb]

I think it might be doable, but not without some refactoring first as I'm not too happy with the API design. Unfortunately I don't have much free time to work on this at the moment.

Also not sure if TarFsFile and ZipFsFile can support Write though...

[GarkGarcia]

Hi there. I'm also involved in the IconPie project.

On regards to the fact that TarFsFile and ZipFsFile cannot support Write, the functionality of Write could be contained in a super trait of Store, so that not all Store implementors are required to support it.

So, it could look something like this:

pub trait Write: Store {
    fn write(&self) -> io::Result<()> {
        // . . .
    }
}