Add constexpr to bit operations

[AndrewSasmito]

Address #241

Re-opened PR with minimal changes to follow the current style

Summary

Restore constexpr support for the clz/ffs bit-counting operations on Windows.

Previously, Windows builds had to remove constexpr from these functions. This also required DynamicBitSet to conditionally mark functions as constexpr depending on the platform.

This change uses std::is_constant_evaluated() to select the appropriate implementation:

• During constant evaluation, use the standard <bit> operations (std::countl_zero / std::countr_zero).
• At runtime on Windows, continue using the MSVC bit-scan intrinsics for their performance.
• Keep the existing compiler-builtin implementations on non-Windows platforms.

As a result, clz and ffs are consistently constexpr across platforms, and the platform-specific CLZ_CONSTEXPR macro can be removed from DynamicBitSet.