std: add string support to totable function.
std: add string support to totable function.

* lib/specl/std.lua (totable): When passed a string, return a
table of each character in the string.

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

gvvaughan/specl