Add `@bound` attribute for loops with statically known bounds.

[cgyurgyik]

calyxir/calyx#534

Not thinking too hard about this, one way I can see it being done is adding another Optional variable to CFor and CWhile called staticBound that can be used in the Calyx backend. @rachitnigam WDYT?

[rachitnigam]

I was thinking that we just generate the bound for CFor loops since they always iterate over a finite domain. The computation should be straightforward: For a for (let i = s..e), we generate @bound(e - s). The one tricky part is that by the time loops get to the calyx backend generator, they've all been turned into CWhile instead. For now, let's add:

case class CWhile(cond: Expr, pipeline: Boolean, body: Command, bound: Option[Int] = None)

And the LowerForLoops pass can ensure that loops that started off as CFor get the right bound. The backend generator can then add @bound(b) for such loops.