feat(stable-api): Add FL_TEST, FL_SET, FL_UNSET for flag manipulation #664

/// Test if specific flags are set on an object (akin to `FL_TEST`).
///
/// # Safety
/// This function is unsafe because it dereferences a raw pointer to get
/// access to underlying RBasic struct. The caller must ensure that the
/// VALUE is a valid pointer to a heap object.
unsafe fn fl_test(&self, obj: VALUE, flags: VALUE) -> VALUE;

/// Test if specific flags are set (returns bool) (akin to `FL_TEST_RAW`).
///
/// # Safety
/// Same as fl_test.
unsafe fn fl_test_raw(&self, obj: VALUE, flags: VALUE) -> bool;

/// Set specific flags on an object (akin to `FL_SET`).
///
/// # Safety
/// This function is unsafe because it modifies internal Ruby object state.
/// The caller must ensure proper synchronization and that the flags being
/// set are appropriate for the object type.
unsafe fn fl_set(&self, obj: VALUE, flags: VALUE);

/// Unset specific flags on an object (akin to `FL_UNSET`).
///
/// # Safety
/// Same as fl_set.
unsafe fn fl_unset(&self, obj: VALUE, flags: VALUE);