Add a mechanism to concatenate several `BitVecValue` at once

[omixann]

Given a collection of &BitVecValue (say, &[BitVecValue]), I'd appreciate a function to build their concatenation that performs a single allocation of the whole required width (bonus points if it has a way of specifying which order to concatenate them, as I need both).

[ekiwi]

That sounds like a good idea. Do you know what kind of impl magic would allow the biggest variety of inputs without compromising the implementation? I guess we would have to be able to iterate over the items twice; Once to get the total length of the output and once to get the actual values. The other option would be to implement this over a mutable BitVecValue that already exists and thus, the users is responsible for making the width match. Then a single iteration would suffice.

[omixann]

Both sound good and IIRC baa already provides both a regular and an in-place variant for several operations. concat_all and concat_all_in_place (for example) could coexist and users would choose the one that better suits their needs.

[ekiwi]

I have been thinking about this a bit more, and here is what I think the first step should be:

• add a assign_slice function
• you can add that right next to the existing assign
• the function should look something like this:

fn assign_slice<'a>(&mut self, value: impl Into<BitVecValueRef<'a>>, lsb: WidthInt) {
      let value = value.into();
      let msb = value.width() - 1 + lsb;
      debug_asser!(self.width() > msb);
      todo!("@omixann: implement the actual assignment")
  }

So then, if you want to concatenate the 2-bit variables a, b, and c, you could do the following:

let mut out = BitVecValue::zero(6);
out.assign_slice(&a, 4);
out.assign_slice(&b, 2);
out.assign_slice(&c, 0);

Does that make sense?

Once you have the assign_slice working, we can always add some further convenience methods to do the concatenation in a single line.