symbol: stringification always returns a symbol's value.

* lib/zmacs/eval.lua: Remove FIXME.
* lib/zile/Symbol.lua (display_variable_value): New function.
If the variable value is a string, then wrap in double quotes
for display.
(symbol.__tostring): Use it.
Adjust all callers that require symbol name to specify exactly
`symbol.name`.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
zile: add missing `string` qualifier to unqualified `gsub`.

* lib/zile/file.lua (create_backup_filename): Add missing
`string.` qualifier.

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