Enhance C++ testing ergonomics of DSLX code

DSLX test capability is limited due to the limited expressibility of the language relative to software languages. In particular, DSLX-based testing of procs is especially awkward. DSLX test features could be added/enhanced à la SystemVerilog or nontrivial testing could be shifted to another language like C++. I'd advocate leaning on C++ to do complex testing to avoid feature bloat in DSLX. Below is an example of what a C++ based testbench could look like:

DSLX to test:

```
struct Foo {
bar: u17,
baz: s44
}

struct ProcState {
a: bool,
b: u32
}

proc MyProc {
c0: chan in;
c1: chan out;
...
}
```

Spitballing what a C++ test might look like below. The generated header defines all the necessary types to make things easy.

```C++
#include "my_proc.h" // auto-generated header

MyProc proc;
Foo input = { .bar=DslxBits<17>(10), .baz=DslxSBits<44>(123) };
proc.GetChannel("c0").Write(input);
proc.Tick();
EXPECT_EQ(proc.GetChannel("c1").Read(), DslxBits<42>(444));
EXPECT_TRUE(proc.GetChannel("c1").empty());
EXPECT_EQ(proc.GetState(), ProcState {.a=DslxBits<1>(true), .b=DslxBits<32>(44)};
```

It might be good to support some of the features in the IR-level Proc runtime [interpreter/proc_runtime.h](https://github.com/google/xls/blob/main/xls/interpreter/proc_runtime.h) like `TickUntilOutput`, `TickUntilBlocked`., etc.