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.

  (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.
Merge pull request #98 from scymtym/fix-symbol-constant-unparsing

Fix unparsing of constant patterns consisting of symbols