[enhancement] Automatically route RAM channels to the top-level

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

Using RAMs can be somewhat cumbersome, particularly in large hierarchies of procs, because RAM rewriting is limited to the top-level proc. As a result, all RAM channels need to be created at the top-level and routed down to the procs that utilize them.

For example, [around ~75% of our channels in the [toplevel ZSTD decoder](https://github.com/google/xls/blob/36bc838b7d0c5a2b3f2e534e749c6f0e140c7075/xls/modules/zstd/zstd_dec.x#L1061) proc are RAM channels

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

There is no alternative. One has to instantiate RAMs only in the top-level and route all channels from top-level to children

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

It would be convenient to remove the need to pass RAM channels down through the proc hierarchy.

* One way to handle this could by by adding annotations to RAM channel declarations, that would cause the automatic routing of channels to the top-level:
```rust
#[expose_port]
let (req_s, req_r) = chan("req");
```

* Since the local RAM usage seems to be a common use case, another approach could be a built-in for creating local RAMs, with their channels automatically routed to the top level. In the upcoming syntax changes, this could be represented as a struct member of the procs.

```rust
proc ProcWithRam {
ram: LocalRam;
}
```
This may be more general as it could be used for example in “bus rewriting” mentioned in #1452