GF.Data.Str: change matchPrefix to agree with the GF book and the run-time system

The matchPrefix function is used in str2strings and the partial evaluator to
compute pre{...} tokens. But unlike the description in the GF book and
the implementation in the run-time system, matchPrefix looked at
the concatenation(!) of all following tokens and not just the next token
when deciding how to compute a pre{...} token.

This is a backwards incompatible change, but it is subtle and probably won't
cause any problems. In particular, the example grammars are unaffected.

src/compiler/GF/Data/Str.hs

@@ -44,8 +44,8 @@ type Ss = [String]
 -- matching functions in both ways
 
 matchPrefix :: Ss -> [(Ss,[String])] -> [String] -> Ss
-matchPrefix s vs t = 
-  head $ [u | let t' = concat (unmarkup t),
+matchPrefix s vs t =
+  head $ [u | t':_ <- [unmarkup t],
               (u,as) <- vs,
               any (`isPrefixOf` t') as]
          ++ [s]