Calculator support for constant value expressions

[themisir]

The grammar allows programs that evaluate constant value, but the parser does not handle it properly. The existing parser implementation didn't handle ~ Term part of the language grammar because its definition was unfolded into Int | Expr.

I implemented the fix by removing _ prefix to disable rule folding and handled Term rules explicitly. The alternative would be to handle Int case individually or introducing a new Const rule that defines Int (and Float as a part of the exercise) and handling it separately.

[ehsanmok]

Good catch! thanks.