cpp_transpiler should emit DSLX constants

[mikex-oss]

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

Currently the cpp_transpiler only converts type definitions for use in C++:

xls/xls/dslx/cpp_transpiler/cpp_transpiler.cc

Lines 107 to 108 in fe11876

	XLS_ASSIGN_OR_RETURN(std::unique_ptr<CppTypeGenerator> generator,
	CppTypeGenerator::Create(def, type_info, import_data));

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.