[enhancement] Support peek() op in XLS

### What's hard to do? (limit 100 words)

Often requested is a peek() op that given a streaming channel, will return the data at the head of the channel (if any) and a valid bit (true if there were data).

While both peek() and recv_non_blocking() bring us out of KPN land, peek() poses a challenge to simulation vs hardware as in simulation we do not backtrack executed instruction when blocking.

For example:

```
(token, data, valid) = peek(token, ch0);
(token, data0) = recv(token, ch0);
```

In hardware, the behavior depends on how peek and recv are scheduled. Let's say that they are scheduled in the same cycle. In this case, once the recv occurs, valid will be true and data will be equal to data0.

In an interpreter, one possible sequence of events is that the channel is empty, and peek executes --> data = 0 and valid = 0. The interpreter then blocks on recv. Once there is data on the channel, recv executes and returns data0.

In order to continue with supporting peek() operation, we should clarify and agree on the behavior of the interpreters, and how the operator is impacted by scheduling.

### Current best alternative workaround (limit 100 words)

This is as opposed to the recv_non_blocking() op which will pop the data at the head of the channel if it exists.

### Your view of the "best case XLS enhancement" (limit 100 words)

One idea that came out of the discussion was to implement peek and receive as a single op/expression -- so we don't need any roll-back functionality in case a receive blocks.

Proposal was something like like the following

1. New expression: peek_and_recv_conditional(\, \, \) : \ { body-expression }
2. body-expression will have available the peek'ed values of each channel and if the channel has data
3. return of the expression will be a tuple: (\, \).
4. All of this expression should be scheduled together.
5. No side-effecting ops in the body.
6. This expression will peek the value of the channels, and determine which receive should happen. If the simulator blocks on a receive, then the simulator will roll-back to the beginning of the expression.