Optimize WCond and ReduceIndex implementations for improved performance and safety

[pluveto]

This PR introduces some optimizations to the WCond and ReduceIndex structs to enhance performance and improve code clarity.

Key Changes:

WCond:

• Introduced some separate variables to streamline the matching process, enhancing readability and maintainability.
• Refactored iterator chains within the Map2 implementation to be more efficient and expressive.

ReduceIndex:

• Removed explicit Clone bounds since T: Copy and U: Copy implicitly include Clone.
• Replaced iter_mut().enumerate() with for i in 0..dst_len loops to reduce iterator overhead and improve loop performance.
• Obtained the destination vector pointer using dst_ptr = dst.spare_capacity_mut().as_mut_ptr() and used the write method to directly insert data. This approach is more explicit and avoids unnecessary type conversions.
• Utilized indices directly within loops instead of creating additional variables.

Next Steps:

• Similar optimizations to other structures.

Regards,

Zijing