Use () instead of {} for the unit

We had been using the empty record `{}` for the unit, but this was
causing problems with conversion to TLA+. We now use the empty tuple,
`()`, which is easily and type-correctly represented in TLA+ as `<<>>`.

quint/io-cli-tests.md

@@ -1130,7 +1130,7 @@ rm xTest.itf.json
 
 <!-- !test out variants in itf -->
 ```
-[{"#meta":{"index":0},"x":{"tag":"None","value":{}}},{"#meta":{"index":1},"x":{"tag":"Some","value":{"#bigint":"1"}}},{"#meta":{"index":2},"x":{"tag":"Some","value":{"#bigint":"2"}}}]
+[{"#meta":{"index":0},"x":{"tag":"None","value":{"#tup":[]}}},{"#meta":{"index":1},"x":{"tag":"Some","value":{"#bigint":"1"}}},{"#meta":{"index":2},"x":{"tag":"Some","value":{"#bigint":"2"}}}]
 ```
 
 ### FAIL on parsing filenames with different casing

quint/src/generated/Quint.g4

@@ -176,8 +176,9 @@ expr:           // apply a built-in operator via the dot notation
         |       'all' '{' expr (',' expr)* ','? '}'                 # actionAll
         |       'any' '{' expr (',' expr)* ','? '}'                 # actionAny
         |       ( qualId | INT | BOOL | STRING)                     # literalOrId
-        //      a tuple constructor, the form tup(...) is just an operator call
+        //      a tuple constructor, the form Tup(...) is just an operator call
         |       '(' expr ',' expr (',' expr)* ','? ')'              # tuple
+        |       '(' ')'                                             # unit
         //      short-hand syntax for pairs, mainly designed for maps
         |       expr '->' expr                                      # pair
         |       '{' recElem (',' recElem)* ','? '}'                 # record