Add dom file system

* Adds A dom filesystem to the list of available fs.
* Adds api to read / write from the dom file system.
* Fixes some error messaging in the code.

Analogous PR in axiom_shell is able to do `cat` and `cp` command using this library code.

The stat format changed since wam.  'source' and 'abilities' are no longer relevant.  The "mode" bitfield should be added, but probably just hardcoded to (Path.mode.r | Path.mode.w | Path.mode.k) for files, and (Path.mode.r | Path.mode.d) for directories.