fs: implement `fs.readfile()` and `fs.writefile()`

The `fs.readfile()` and `fs.writefile()` functions allow efficient reading
and writing of whole files, reducing the required boilerplace code compared
to using the `open()`/`read()`/`write()`/`close()` API.

 - `fs.readfile()` takes two arguments; the path to open and an optional
   limit value. If limit is omitted, `null` or negative, the entire file
   contents are read, otherwise the specified amount of bytes. Returns the
   read file contents as string or `null` on error.

 - `fs.writefile()` takes three arguments; the path to open/create, the
   contents to write and an optional limit value. If limit is omitted, the
   entire content is written, otherwise just the specified amount of bytes.
   Non-string content arguments are internally converted to strings, `null`
   is treated as empty string. Returns the amount of bytes written or `null`
   on error.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

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