Move parsing of 'case' out of simpleExpr

Even if this is possible, using it as an application unbracketed would
be unreadable! This speeds up parsing of nested case blocks massively.
(Or should I say "unslows down").

src/Idris/ParseExpr.hs

@@ -181,6 +181,7 @@ InternalExpr ::=
   | Let
   | RewriteTerm
   | Pi
+  | CaseExpr
   | DoBlock
   ;
 @
@@ -199,6 +200,7 @@ internalExpr syn =
      <|> rewriteTerm syn
      <|> try(pi syn)
      <|> doBlock syn
+     <|> caseExpr syn
      <|> simpleExpr syn
      <?> "expression"
 
@@ -263,7 +265,6 @@ SimpleExpr ::=
   | 'refl' ('{' Expr '}')?
   | ProofExpr
   | TacticsExpr
-  | CaseExpr
   | FnName
   | List
   | Comprehension
@@ -290,7 +291,6 @@ simpleExpr syn =
         <|> do reserved "elim_for"; fc <- getFC; t <- fnName; return (PRef fc (SN $ ElimN t))
         <|> proofExpr syn
         <|> tacticsExpr syn
-        <|> caseExpr syn
         <|> do reserved "Type"; return PType
         <|> try (do c <- constant
                     fc <- getFC

src/Idris/ParseHelpers.hs

@@ -260,7 +260,8 @@ operatorLetter = oneOf opChars
 -- | Parses an operator
 operator :: MonadicParsing m => m String
 operator = do op <- token . some $ operatorLetter
-              when (op == ":") $ fail "(:) is not a valid operator"
+              when (op `elem` [":", "=>", "->", "<-", "=", "?="]) $ 
+                   fail $ op ++ " is not a valid operator"
               return op
 
 {- * Position helpers -}