PGF service: add commands c-abstrtree and c-parsetree

These are the C runtime variants of the absttree and parsetree commands. They
work in the same way, but support fewer rendering options.

src/server/PGFService.hs

@@ -135,6 +135,8 @@ cpgfMain qsem command (t,(pgf,pc)) =
     "c-lookupmorpho"-> out t=<< morpho # from1 % textInput
     "c-flush"       -> out t=<< flush
     "c-grammar"     -> out t grammar
+    "c-abstrtree"   -> outputGraphviz=<< C.graphvizAbstractTree pgf # tree
+    "c-parsetree"   -> outputGraphviz=<< C.graphvizParseTree . snd # from1 %tree
     "c-wordforword" -> out t =<< wordforword # input % to
     _               -> badRequest "Unknown command" command
   where