Pool and Allocator are difficult to use together

[pchickey]

ngx::core::Pool impls the allocator-api2 Allocator trait, which makes allocations with &self methods.

However, all of the allocations made through Pool's own impl methods take &mut self.

In practice, this means that if I have an ngx_pool_t pointer and I have one API that uses the pool via Allocator (e.g. NgxString::new_in) and another that uses Pool's impl methods directly (e.g. https://github.com/nginx/nginx-acme/blob/main/src/net/resolver.rs#L138), I have to construct two different Pools from the same raw pointer.

I don't think the Allocator impl is unsound, so I propose the best way to solve this is to downgrade all of Pool's impl methods from &mut self to &self.