Slight for loop gas optimization

[redsh4de]

• Changed increment operations to use pre-increment instead of post-increment to save an extra operation
• for loop iterator incrementing is moved into an unchecked block to skip overflow checks and save gas - given that i has a maximum limit set by the loop, there are no downsides for this approach

[mudgen]

@redsh4de I appreciate this. I found one bug in the pull request. Please fix and I can merge. Thanks for your help.

[redsh4de]

That should take care of it! @mudgen