symbol: tostring returns symbol name, not value.

Previously, printing symbol lists for debugging purposes was
peculiarly unhelpful, just to save a little code when evaluating
a symbol from ZLisp.  We don't use the evaluation short-cut any
more, so let's print the symbol name from tostring so that
symbol lists work as expected.
* lib/zile/Symbol.lua (display_variable_value): Remove.
(metatable.__tostring): Return symbol name.

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