Evaluate: A struct version of allocator

https://internals.rust-lang.org/t/why-bring-your-own-allocator-apis-are-not-more-popular-in-rust/14494/3?u=heroickatora

It seemed interesting to me because this has the distinct advantage of not necessitating a type parameter, which was also a 'revelation' when dealing with forward compatible ways to design markers for a type being transmutable from bytes (compare [my blog post on the topic](https://www.hardmo.de/article/2021-03-14-zst-proof-types.md#zsts-as-marker-trait-replacements)). This comes at the cost of extra layout size of all containers containing such an allocator.