[Tolk] Allow cell and slice be valid identifiers

They are not keywords anymore.
> var cell = ...;
> var cell: cell = ...;
Motivation: in the future, when structures are implemented, this obviously should be valid:
> struct a { ... }
> var a = ...;
Struct fields will also be allowed to have names int/slice/cell.

tolk-tester/tests/assignment-tests.tolk

@@ -14,6 +14,18 @@ fun autoInferIntNull(x: int) {
     return x;
 }
 
+fun typesAsIdentifiers(builder: builder) {
+    var int = 1;
+    var cell = builder.endCell();
+    var slice = cell.beginParse();
+    {
+        var cell: cell = cell;
+        var tuple: tuple = createEmptyTuple();
+        var bool: bool = tuple.tupleAt<int>(0) > 0;
+    }
+    return int;
+}
+
 fun main(value: int) {
     var (x: int, y) = (autoInferIntNull(value), autoInferIntNull(value * 2));
     if (x == null && y == null) { return null; }

tolk-tester/tests/invalid-catch-1.tolk

@@ -1,12 +1,12 @@
 fun main() {
     try {
 
-    } catch(int, arg) {}
+    } catch(if, arg) {}
     return 0;
 }
 
 /**
 @compilation_should_fail
-@stderr expected identifier, got `int`
-@stderr catch(int
+@stderr expected identifier, got `if`
+@stderr catch(if
  */

tolk-tester/tests/invalid-declaration-2.tolk

@@ -4,5 +4,5 @@ fun main(int): int {
 
 /**
 @compilation_should_fail
-@stderr expected parameter name, got `int`
+@stderr expected `: <parameter_type>`, got `)`
 */

tolk-tester/tests/invalid-declaration-4.tolk

@@ -4,5 +4,5 @@ fun main() {
 
 /**
 @compilation_should_fail
-@stderr probably, you use FunC-like declarations; valid syntax is `var x: int = ...`
+@stderr expected `;`, got `x`
 */

tolk-tester/tests/method_id.tolk

@@ -4,6 +4,8 @@ fun foo1(): int { return 111; }
 fun foo2(): int { return 222; }
 @method_id(10)
 fun foo3(): int { return 333; }
+@method_id(11)
+fun slice(slice: slice): slice { return slice; }
 fun main(): int { return 999; }
 
 /**