21: Improve solution for Rust track exercise "Beer Song" (remove unnecessary `f64` casts/conversions).

Dependencies

• [2] Z4YP3EEK 19: Improve performance of solution for Rust track exercise "Beer Song" at the cost of duplication.
• [3] 6DOVDD66 15: Add solution for Rust track exercise "Beer Song".
• [4] QK6XE5XF 13: Add Rust track exercises "Beer Song", "Proverb", "Difference Of Squares", "Sum Of Multiples", "Grains", "Prime Factors", "Armstrong Numbers" and "Reverse String".
• [5] 7NUHCTMK 16: Apply `cargo fmt` and `cargo clippy` to the solution for Rust track exercise "Beer Song". Also add previous versions.

Change contents

• replacement in rust/beer-song/src/lib.rs at line 15
  [3.2117]→[3.2117:2156](∅→∅)
  − fn integer_length(a: usize) -> usize {
  [3.2117]

  [3.2156]
  + fn digits(a: usize) -> usize {
• replacement in rust/beer-song/src/lib.rs at line 19
  [3.2257]→[2.639:722](∅→∅),[2.722]→[3.3678:3819](∅→∅),[3.3678]→[3.3678:3819](∅→∅)
  − // Might not work properly if results of `powi`s don't fit into the `usize` casts.

  − fn integer_range_length(start: usize, end: usize) -> usize {

  − let (start_digits, end_digits) = (integer_length(start), integer_length(end));
  [3.2257]

  [3.12075]
  + // Present conversions and type casts could be problematic for certain inputs.

  + fn range_digits(start: usize, end: usize) -> usize {

  + let (start_digits, end_digits) = (digits(start), digits(end));
• replacement in rust/beer-song/src/lib.rs at line 23
  [3.12091]→[3.12091:12219](∅→∅)
  − (10_f64.powi(start_digits.try_into().unwrap()) - 1.0) as usize,

  − 10_f64.powi((end_digits - 1).try_into().unwrap()) as usize,
  [3.12091]

  [3.12219]
  + 10_usize.pow(start_digits.try_into().unwrap()) - 1,

  + 10_usize.pow((end_digits - 1).try_into().unwrap()) as usize,
• replacement in rust/beer-song/src/lib.rs at line 26
  [3.12223]→[3.12223:12368](∅→∅)
  − (integer_length(a + 1)..=integer_length(b - 1)).fold(0, |acc, elem| {

  − acc + 9 * elem * (10_f64.powi((elem - 1).try_into().unwrap()) as usize)
  [3.12223]

  [3.12368]
  + (digits(a + 1)..=digits(b - 1)).fold(0, |acc, elem| {

  + acc + 9 * elem * 10_usize.pow((elem - 1).try_into().unwrap())
• replacement in rust/beer-song/src/lib.rs at line 50
  [3.12494]→[3.12494:12539](∅→∅)
  − + integer_range_length(3, s as usize)
  [3.12494]

  [3.12539]
  + + range_digits(3, s as usize)