CellBounds: Handle comparisons with builder.bits() and builder.refs()

[jubnzv]

If there are comparisons involving these method calls in the dataflow, we should not consider data and references added later in the dataflow as rule violations.

[jubnzv]

Using this approach we could also generate warnings for Cell arguments of unknown size:

fun test(c: Cell /* unknown size */) {
  let s = c.asSlice();

  s.loadBool(); // Bad: Possible cell underflow

  if (s.bits() > 1) {
    s.loadBool(); // OK
  }
}