feat: Return values

[JonathanWoollett-Light]

Summary of the PR

Allowing a return value for the event closures. This allows for proper error propagation.

Benchmarks

process_basic           time:   [9.8030 µs 9.8052 µs 9.8077 µs]
                        change: [-26.377% -26.322% -26.261%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 18 outliers among 200 measurements (9.00%)
  7 (3.50%) high mild
  11 (5.50%) high severe

process_with_arc_mutex  time:   [12.007 µs 12.015 µs 12.025 µs]
                        change: [-11.346% -11.213% -11.093%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 200 measurements (3.50%)
  4 (2.00%) high mild
  3 (1.50%) high severe

process_with_inner_mut  time:   [10.549 µs 10.552 µs 10.556 µs]
                        change: [-17.740% -17.639% -17.534%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 21 outliers among 200 measurements (10.50%)
  8 (4.00%) high mild
  13 (6.50%) high severe

process_dynamic_dispatch
                        time:   [20.868 µs 20.873 µs 20.878 µs]
                        change: [+12.268% +12.435% +12.596%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 29 outliers among 200 measurements (14.50%)
  7 (3.50%) high mild
  22 (11.00%) high severe

process_dispatch_few_events
                        time:   [552.18 ns 552.28 ns 552.39 ns]
                        change: [-22.801% -22.706% -22.620%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 26 outliers among 200 measurements (13.00%)
  1 (0.50%) low mild
  10 (5.00%) high mild
  15 (7.50%) high severe

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR are signed (with git commit -s), and the commit
  message has max 60 characters for the summary and max 75 characters for each
  description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.