Rewrite sprite collision functions

[aatle]

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The current sprite collision functions code has some code repetition. This implementation structures it differently, which has a few effects.

• If dokill is true in spritecollide or groupcollide, the sprites are killed after all colliding sprites have been determined, instead of during iteration (should be unnoticeable)
• As a consequence of the previous thing, in these cases, iteration over group.sprites() is replaced by iteration over group itself, resulting in the following:
  • Avoids a list copy of the group, improving performance slightly in around 1/8 of possible cases
  • It is no longer an error to use a non-group iterable of sprites here (already no error if dokill is False)
    • I advocate for typing the sprite collide functions as supporting all iterables of sprites, instead of just sprite groups
  • "Old-style" groups that don't support iteration raise error now here (already errors if dokill is False)
• It is now easy to implement an ignore_self parameter, e.g., with the new structure

Performance tested in 3.13.0, no regression (similar performance). Renamed a few local variables for clarity.