[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/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 `)`
 */