Lexer.x & Parser.y: add a partial parser for terms

Lexer.x: Change the parser monad type P to allow the remaining input to
	 be returned after a partial parse. Add function

           runPartial :: P t -> String -> Either (Posn, String) (String, t)

Parser.y: Add a partial parser pTerm for nonterminal Exp1.
          Re-export runPartial.

src/compiler/GF/Grammar/Parser.y

@@ -2,9 +2,10 @@
 {
 {-# OPTIONS -fno-warn-overlapping-patterns #-}
 module GF.Grammar.Parser
-         ( P, runP
+         ( P, runP, runPartial
          , pModDef
          , pModHeader
+         , pTerm
          , pExp
          , pTopDef
          , pBNFCRules
@@ -30,6 +31,7 @@ import PGF(mkCId)
 %name pModDef ModDef
 %name pTopDef TopDef
 %partial pModHeader ModHeader
+%partial pTerm Exp1
 %name pExp Exp
 %name pBNFCRules ListCFRule
 %name pEBNFRules ListEBNFRule