Separate Write methods into their own interface

Write is pretty basic, but after some thought I've come to believe we don't want write multiplexing. So Writable should get it's own interface:

```go
type WriteableFilesystem interface {
  Put(ctx context.Context, f File) (string, error)
}
```

This way `mix` can _not_ implement this interface, which'll show users they must first demux before writing.