Improve example solution of circular-buffer

[senekor]

The trait bounds Default + Clone are semantically incorrect. A proper circular buffer needs to handle all kinds of elements.

The reason these bounds are here is because it allows to avoid unsafe while enjoying the most efficient memory layout.
However, there is another performance downside:
The implementations of Default and Clone may be expensive (e.g. cause heap allocations.)

As came up in this discussion, there are other weird side effects, for example for Rc which has a non-standard implementation of Clone: #1652

The approach I chose here get's rid of the trait bounds and wraps every element in an Option.
In the worst case, this may lead to twice the memory consumption. (e.g. Option<u64> takes 16 bytes because of alignment and padding) This approach is semantically correct, but not the most performant.

The most performant solution would be to use unsafe. I'd like to avoid that in example solutions.

[senekor]

Current dependencies on/for this PR:

• main
  • PR Add test generator #1722
    • PR Use todo instead of unimplemented #1723
      • PR Remove test_ prefix from tests #1727
        • PR Add tera filter to_hex for use in test templates #1732
          • PR Symlink problem-specifications for easy reference #1737
            • PR Sync pascals-triangle with problem-specifications #1743
            • PR Sync custom-set with problem-specifications #1741
            • PR doubly-linked-list: Fix inaccurate instructions #1740
            • PR Sync two-bucket with problem-specifications #1739
            • PR Sync variable-length-quantity with problem-specifications #1731
            • PR Sync word-count with problem-specifications #1729
        • PR Sync wordy with problem-specifications #1728
        • PR Decimal: Compare different numbers of digits #1726
        • PR Improve test order of low-power-embedded-game #1725
        • PR Improve example solution of circular-buffer #1724 👈

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