Make real type works

Otherwise for the following code:

```
node test (i: real) returns (o: real);
let
    o = 0.0;
tel
```

was experiencing:

```
Fatal error: exception Stdlib.Parsing.Parse_error
Raised at Stdlib__Parsing.yyparse.loop in file "parsing.ml", line 139, characters 8-25
Called from Stdlib__Parsing.yyparse in file "parsing.ml", line 165, characters 4-28
Re-raised at Stdlib__Parsing.yyparse in file "parsing.ml", line 184, characters 8-17
Called from Parser.main in file "parser.ml" (inlined), line 1110, characters 4-44
Called from Main in file "main.ml", line 70, characters 16-68
```

Note that `%token REAL` doesn't help to solve this error, but it doesn't seem to be any reason for not having it.

src/lexer.mll

@@ -14,6 +14,7 @@
       ("returns", RETURNS);
       ("var", VAR);
       ("int", TYP(Ast.TInt));
+      ("real", TYP(Ast.TReal));
       ("bool", TYP(Ast.TBool));
       ("<=", CMP_le);
       (">=", CMP_ge);

src/parser.mly

@@ -118,6 +118,7 @@
 %token COLON
 %token BOOL
 %token INT
+%token REAL
 %token LET
 %token TEL
 %token NODE