cpp_transpiler should emit DSLX constants

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

Currently the cpp_transpiler only converts type definitions for use in C++: https://github.com/google/xls/blob/fe11876713d1710ae0e7aab974dbc1197221475b/xls/dslx/cpp_transpiler/cpp_transpiler.cc#L107-L108

It would be useful to also convert constants so that the C++ is aware of the parametrization of DSLX types.

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

In some cases, you could make fake types in the DSLX library that would generate what you want as a side effect. For example:

```
const FOO = u32:32; // constant I care about

type FooBits = bits[FOO];
```

would generate something like

```
using FooBits = uint32_t;
constexpr int64_t kFooBitsWidth = 32; // hurray!
```

Alternatively, the constants can be redefined in the consumer C++ library, but this creates multiples sources of truth.

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

Emit DSLX `const` definitions in the C++ generated library.