PGF run-time library: function names in BracketedString (experimental)

+ Make room for function names in the BracketedString data structure.
+ Fill in function names when linearizing an abstract syntax tree to a
  BracketedString.
+ Fill in wildCId when it is not obvious what the function is.
+ Function bracketedLinearize: for compatibility with the other linearization
  functions, return Leaf "" instead of error "cannot linearize".
+ Export flattenBracketedString from module PGF.
+ PGFServce: make function names available in the JSON representation of
  BracketedString.

src/runtime/haskell/PGF.hs

@@ -58,7 +58,7 @@ module PGF(
            showPrintName,
            
            BracketedString(..), FId, LIndex, Token,
-           Forest.showBracketedString,
+           Forest.showBracketedString,flattenBracketedString,
 
            -- ** Parsing
            parse, parseAllLang, parseAll, parse_, parseWithRecovery,