Allow basic computation and references to other constants in CONST definition

Right now, a `CONST` must be defined as either a number or divert target.

```
CONST MyConstant = 5
```

However, it's frequently useful to define constants in reference to other constants.

```
CONST SunriseTime = 6
CONST Breakfast = SunriseTime // Constant equals an existing constant
CONST Lunch = Breakfast + 6 // Constant equals a constant expression
```

So this request is two-fold:

- Support for defining a constant as the value of another constant
- Support for defining a constant by a constant expression (arithmetic on other constants)

This could all be resolved at compile-time, so it may actually be purely an inklecate feature with no ink/inkjs/etc. implications.