Clean up code-base from external review

[Kronos3]

There aren't any breaking API changes here, just correctness and maintenance cleanups we found when reviewing the code-base.

[github-actions]

CoreMark Benchmark Results

Current Score: 236.500
Baseline Score (main): 241.371
Difference: -4.871 (-2.02%)

[github-actions]

Code Coverage Report

Current Coverage: 95.50%
Baseline Coverage (main): 95.50%
Difference: -0.01%

[sylvesterkaczmarek]

The new try_new() path propagates Box::new(f) allocation failure, but get_call() still allocates Box::new(placeholder).unwrap() on every invocation. Under a fallible/custom allocator that can still panic at runtime after construction succeeded. Could the placeholder be allocation-free/preallocated, or can get_call() propagate AllocError too?