Add the ability to use a specific Allocator

[Limeth]

I have been using Slint for a project built for the ESP32-S3 SoC. One issue I ran into is that Slint uses a lot of memory.
The ESP32-S3 has 512 KiB of internal RAM, and up to 16 MiB of external SPIRAM. I considered making my global allocator allocate in the external SPIRAM, but that is currently likely to cause undefined behavior on the ESP32-S3 because of unsupported atomic operations in the PSRAM.
Therefore, I decided to make my global allocator allocate in the internal RAM, and use a custom allocator with the allocator_api nightly Rust feature for large allocations without atomics.
Since Slint uses a lot of memory, I would like to make it use the PSRAM as much as possible. With some help from the Slint chat, I have been able to patch Slint to use a custom allocator for some allocations.
However, it would be nice if there was a more general, well supported solution.