Improve Variant and SafeArray implementations

[ohadravid]

Safety:

• In safearray: Now SafeArrayAccessor::new and safe_array_to_vec{_of_strings} are unsafe, as it is the caller's responsibility to pass valid arrays.
• SafeArrayAccessor now exposes an iterator over the array (and not a slice), based on the data pointer+element count (vs the lower/upper bounds)

Other changes:

• Variant::from_variant will now use windows-rs's VARIANT::to_string