Forward declaration syntax for data

Allow:
```newt
data Foo : U
```
as a forward declaration for data. (The `Foo : U` syntax still works for
now.)

src/Lib/Parser.newt

@@ -615,9 +615,10 @@ parseData = do
   -- commit when we hit ":"
   name <- try $ (keyword "data" *> (uident <|> ident <|> token MixFix) <* keyword ":")
   ty <- typeExpr
-  keyword "where"
+  Just _ <- optional (keyword "where")
+    | _ => pure $ Data fc name ty Nothing
   decls <- startBlock $ manySame $ parseSig
-  pure $ Data fc name ty decls
+  pure $ Data fc name ty (Just decls)
 
 nakedBind : Parser Telescope
 nakedBind = do

src/Lib/ProcessDecl.newt

@@ -229,7 +229,7 @@ processClass ns classFC nm tele decls = do
 
   log 1 $ \ _ => "tcon type \{render 90 $ pretty tcType}"
   log 1 $ \ _ => "dcon type \{render 90 $ pretty dcType}"
-  let decl = Data classFC nm tcType (TypeSig classFC (dcName :: Nil) dcType :: Nil)
+  let decl = Data classFC nm tcType (Just $ TypeSig classFC (dcName :: Nil) dcType :: Nil)
   log 1 $ \ _ => "Decl:"
   log 1 $ \ _ => render 90 $ pretty decl
   processDecl ns decl
@@ -381,7 +381,7 @@ processShortData ns fc lhs sigs = do
   (nm,args) <- getArgs lhs Nil
   let ty = foldr mkPi (RU fc) args
   cons <- traverse (mkDecl args Nil) sigs
-  let dataDecl = Data fc nm ty cons
+  let dataDecl = Data fc nm ty (Just cons)
   log 1 $ \ _ => "SHORTDATA"
   log 1 $ \ _ => "\{render 90 $ pretty dataDecl}"
   processDecl ns dataDecl
@@ -504,7 +504,7 @@ processRecord ns recordFC nm tele cname decls = do
 
   log 1 $ \ _ => "tcon type \{render 90 $ pretty tcType}"
   log 1 $ \ _ => "dcon type \{render 90 $ pretty dcType}"
-  let decl = Data recordFC nm tcType (TypeSig recordFC (dcName :: Nil) dcType :: Nil)
+  let decl = Data recordFC nm tcType (Just $ TypeSig recordFC (dcName :: Nil) dcType :: Nil)
   log 1 $ \ _ => "Decl:"
   log 1 $ \ _ => render 90 $ pretty decl
   processDecl ns decl
@@ -544,5 +544,7 @@ processDecl ns (DCheck fc tm ty) = processCheck ns fc tm ty
 processDecl ns (Class classFC nm tele decls) = processClass ns classFC nm tele decls
 processDecl ns (Instance instfc ty decls) = processInstance ns instfc ty decls
 processDecl ns (ShortData fc lhs sigs) = processShortData ns fc lhs sigs
-processDecl ns (Data fc nm ty cons) = processData ns fc nm ty cons
+processDecl ns (Data fc nm ty (Just cons)) = processData ns fc nm ty cons
+-- TODO distinguish from function signatures
+processDecl ns (Data fc nm ty Nothing) = processTypeSig ns fc (nm :: Nil) ty
 processDecl ns (Record recordFC nm tele cname decls) = processRecord ns recordFC nm tele cname decls

src/Lib/Syntax.newt

@@ -76,17 +76,15 @@ instance HasFC Raw where
 
 data Import = MkImport FC Name
 
-
-
 Telescope : U
 Telescope = List (BindInfo × Raw)
 
-
 data Decl
   = TypeSig FC (List Name) Raw
   | FunDef FC Name (List (Raw × Maybe Raw))
   | DCheck FC Raw Raw
-  | Data FC Name Raw (List Decl)
+  -- TODO maybe add Telescope (before `:`) and auto-add to constructors...
+  | Data FC Name Raw (Maybe $ List Decl)
   | ShortData FC Raw (List Raw)
   | PType FC Name (Maybe Raw)
   | PFunc FC Name (List String) Raw String
@@ -247,7 +245,8 @@ instance Pretty Decl where
       prettyPair : Raw × Maybe Raw → Doc
       prettyPair (a, Nothing) = pretty a
       prettyPair (a, Just b) = pretty a <+> text "=" <+> pretty b
-  pretty (Data _ nm x xs) = text "data" <+> text nm <+> text ":" <+>  pretty x <+> (nest 2 $ text "where" </> stack (map pretty xs))
+  pretty (Data _ nm x Nothing) = text "data" <+> text nm <+> text ":" <+>  pretty x
+  pretty (Data _ nm x (Just xs)) = text "data" <+> text nm <+> text ":" <+>  pretty x <+> (nest 2 $ text "where" </> stack (map pretty xs))
   pretty (DCheck _ x y) = text "#check" <+> pretty x <+> text ":" <+> pretty y
   pretty (PType _ nm ty) = text "ptype" <+> text nm <+> (maybe empty (\ty => text ":" <+> pretty ty) ty)
   pretty (PFunc _ nm Nil ty src) = text "pfunc" <+> text nm <+> text ":" <+> nest 2 (pretty ty <+> text ":=" <+/> text (show src))