Extract `wordsIndex` to calculate the index of words in a `uint64`, and use it to replace `i % wordSize`. In addition, change the `InsertAt` function to reduce one bit operation.
Extract `wordsIndex` to calculate the index of words in a `uint64`, and use it to replace `i % wordSize`. In addition, change the `InsertAt` function to reduce one bit operation.

willf/bitset