Move NSym out of NAtom

An NSym doesn't evaluate to itself and never appears in an NValue, so it
should not be an atom.
Fix pretty printing precedence of special operator
Implement indented strings + uri + SPATH

We now parse till line 10005 of all-packages.nix
Fix function application vs operator precedence

We now parse all of all-packages.nix! There are probably still
some parse bugs to catch, though.