root: add File type

[enricozb]

adds the File type and basic functions open and .read_bytes. Example usage looks like:

use #root::IO::File;

pub fn main(&io: &IO) {
  let file = File::open(&io, "Cargo.toml").assume();
  let bytes = file.read_bytes().assume();
  let content = String(bytes.map(fn*(b: N32) { b as Char }));

  io.println("{content}");

  file.close(&io);
}