zmacs: improve setq special-form.

* lib/zile/zlisp.lua (lex): Rename the "string" token type to
"literal".  Also recognise numbers, t and nil, and return those
as "literal" tokens too.
* lib/zmacs/eval.lua (evaluate_expression): Work properly when
evaluating literals, quoted nodes, variable references and
function calls.

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