[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/lexer.h

@@ -118,14 +118,6 @@ enum TokenType {
   tok_if,
   tok_else,
 
-  tok_int,
-  tok_cell,
-  tok_bool,
-  tok_slice,
-  tok_builder,
-  tok_continuation,
-  tok_tuple,
-  tok_void,
   tok_arrow,
   tok_as,