stdlib: import prettyprint improvements from upstream.
stdlib: import prettyprint improvements from upstream.

* lib/specl/std.lua (render): Output table keys in order.
(prettytostring): Omit quoting for table keys that are already
valid symbol names.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/specl