Fix unparsing of constant patterns consisting of symbols

Quoted symbols did not roundtrip through
```
PARSE-PATTERN -> UNPARSE-PATTERN
```
because they were unparsed without `quote`. I.e.
```cl
(unparse-pattern (parse-pattern ''x))
=> X instead of (QUOTE X)
```
This change causes all symbols except keywords, `t` and `nil` to be unparsed
with `quote`.