Add missing file operations

[talex5]

The FS API is currently missing lots of functions that are in OCaml's Unix:

• stat and lstat (you can stat an open file though)
• fsync (Add Eio.File.{seek,sync,truncate} #626)
• lseek (Add Eio.File.{seek,sync,truncate} #626)
• ftruncate (Add Eio.File.{seek,sync,truncate} #626) (and truncate?)
• map_file
• link, symlink
• readlink
• realpath
• Implement chmod support #764
• chown
• access

Also, would be good to provide helpers for:

• Creating temporary files (requested by @adatario).
• Add Path.mkdirs and Path.split #625

[adatario]

Similarily it might be nice to expose something that would allow the creation of temporary files without going trough env#fs.

Maybe by exposing a temp_dir path in the Stdenv that uses Unix.get_temp_dir_name ?

[yawaramin]

Or using Filename.get_temp_dir_name to avoid Unix dependency.

[avsm]

The only one of this list that I'm a little wary of adding to Eio without some thought is map_file. It's almost never the right answer to memory map a large amount of data, since you lose control of when the process blocks in the memory subsystem. I wonder if we can build something with a similar interface using OCaml's custom array indexing operators, but that goes via uring/pread calls instead of mmap.