message: Document why alloca with a static type isn't worth it
  Summary: I did the benchmarks, and it's negligible, so keep the more flexible variant.
diff: @@ -25,6 +25,9 @@ use std::{
  /// so define our own wrapper.
  /// We use a single continuous buffer. When it needs upgrading, we double it and keep the old one around.
  pub(crate) struct Alloca {
  + // An alternative design would be to bake the <T> into the type, so all allocations are of the same type.
  + // Benchmarking that, even if the most optimistic scenario (just reallocating a single element)
  + // the performance difference is only 2%, so keep the flexibility of not needing to predeclare the type.
  alloc: Cell<*mut u8>,
  end: Cell<*mut u8>,
  last_size: Cell<usize>,
  