Implement `chmod` support

[patricoferris]

A specific issue tied to #510 -- I started this work over at https://github.com/patricoferris/eio/tree/fchmodat -- Anyone who is interested is more than welcome to take the work started there and get it over the line.

Initially this could be for Unix-y backends, but having Windows support would be great. We might take some inspiration from what the Unix module in the compiler is doing: https://github.com/ocaml/ocaml/blob/trunk/otherlibs/unix/chmod.c

[create2000]

Hello @patricoferris I would like to take the work. Thank you

[patricoferris]

Hey @create2000 -- thanks for the enthusiasm! Let's work on #766 and get that merged before picking up something else :))

[create2000]

Hello @patricoferris -- is this still available?

[webbunivAdmin]

hello @patricoferris could you assign this to me

[patricoferris]

Hey @webbunivAdmin, thanks for coming to the issue from discord, please do let me know if you need any help.