Minor tidy up

[Vectorized]

Changes:

• Lint
• Refactored _boolToUint256 with the shifting into a _nextInitializedFlag function.
  The line is too long and triggers my OCD,
  and we can put the duplicated bitwise shifting stuff into the function.
• Break up some long comments into multiple lines.
• Changed _currentIndex = startTokenId + quantity; to _currentIndex = end;. Saves 1 gas.

Comments:

After some consideration, I've decided not to use assembly for the mint loop for now.
I know it saves 30 gas per iteration (0.4% savings for a batch of 10),
but it is hard to make it look pretty with all the magic numbers.
Anyway, ETH is cheap now.