Add a non-overloaded "::" operator to assign Cduce types to OCaml polymorphic values.

This allows to write `Stdlib.List.map :: {Int Bool}`. The previous syntax,
`Stdlib.List.map with { Int Bool }` is ambiguous with the `match ... with` and needs
arbitrary look-ahead. It can still be written, with explicit parentheses.

lang/parser/parser.mly

@@ -181,6 +181,7 @@ let id_dummy = U.mk "$$$"
 %left SETMINUS "//" "/@" "/"
 %nonassoc ":"
 %nonassoc "!" unary_op
+%left "::"
 %left "."
 %nonassoc "," ")"
 
@@ -889,6 +890,7 @@ no_seq_expr:
 ;
 
 simple_expr:
+| e = simple_expr "::" "{" l = nonempty_list(pat) "}" { exp $sloc (TyArgs(e, l)) }
 | e = simple_expr "." l = ident_or_keyword { exp $sloc (Dot (e, label l)) }
 | "(" e = expr  l = with_annot ")" { exp $sloc (TyArgs (e, l)) }
 | "(" l = separated_nonempty_list(",", multi_expr) ")" { exp $sloc (tuple l) }
@@ -923,7 +925,7 @@ simple_expr:
 ;
 
 with_annot:
-| "with" "{" l= list(pat) "}" { l }
+| "with" "{" l= nonempty_list(pat) "}" { l }
 ;