Remove unnecessary unit struct fields from Units enum variants

[Turbo87]

The unit struct fields in Units enum variants were never accessed - they were only used for construction and immediately discarded in pattern matching.

Changes:

• Convert tuple-like enum variants (e.g., steradian(steradian)) to unit-like variants (e.g., steradian)
• Update pattern matching to use simpler syntax without wildcards
• Update ALL_UNITS initialization to use unit-like syntax
• Update tests to construct enum variants without passing unit structs

Benefits:

• Simpler, clearer API
• Smaller enum variants (no tuple wrapper)
• Less confusion about unused fields

Breaking change: External code constructing Units enum variants must use unit-like syntax instead of tuple-like syntax.

Fixes #539

[iliekturtles]

Thanks for the PR. I rebased on top of the latest master and pushed to force Github checks to show up. The PR must have been submitted during the Github actions outage, because checks weren't originally available.

[Turbo87]

checks weren't originally available

I think these days they only run by default if the PR author has already committed to the repo before :)

[iliekturtles]

Normally I get an approval button for first-time contributors, but it was missing for your PRs!

Hyperfine results, 1.03 +/- 0.02 faster compile times with the PR using rustc 1.93.0. (EDIT, re-ran on a quiet system)

$ hyperfine "touch uom/src/lib.rs; cargo build --manifest-path=uom/Cargo.toml" "touch uom-remove-units/src/lib.rs; cargo build --manifest-path=uom-remove-units/Cargo.toml"
Benchmark 1: touch uom/src/lib.rs; cargo build --manifest-path=uom/Cargo.toml
  Time (mean ± σ):      3.600 s ±  0.045 s    [User: 3.158 s, System: 0.401 s]
  Range (min … max):    3.552 s …  3.702 s    10 runs

Benchmark 2: touch uom-remove-units/src/lib.rs; cargo build --manifest-path=uom-remove-units/Cargo.toml
  Time (mean ± σ):      3.508 s ±  0.034 s    [User: 3.057 s, System: 0.387 s]
  Range (min … max):    3.459 s …  3.563 s    10 runs

Summary
  touch uom-remove-units/src/lib.rs; cargo build --manifest-path=uom-remove-units/Cargo.toml ran
    1.03 ± 0.02 times faster than touch uom/src/lib.rs; cargo build --manifest-path=uom/Cargo.toml