Feature request: secrecy: Provide support for custom Allocator

[stefanschmidt77]

Hi,

I want to store a box allocated with sodium_alloc::SodiumAllocator inside a SecretBox, so I can provide additional memory protection for the boxed data.
As the allocator_api is still a nightly-only feature, I would guess it should be gated behind an optional feature.

Let me know your thoughts and the chances of getting an implementation merged.

Best regards,
Stefan

[stefanschmidt77]

On rereading the docs on sodium_free(), using a custom allocator on Box looks like a more complete solution, except maybe for the ExposeSecret trait's auditabilty.

[tony-iqlusion]

I don't think there's any way to expose custom allocators without making the whole crate nightly-only