I can now successfully read the file from disk into RAM!

Instead of uefi::fs, I again use the SimpleFileSystem protocol directly.
This is so I can allocate the buffer myself using allocate_pages, bypassing the heap. This unfortunately introduces another unsafe block, but that’s a small price to pay.
After that, the file also successfully reads into RAM. read contains logic to read in 1KiB blocks, to bypass bugs in some firmwares.
Next stop: Mounting the RAM Disk, using my self-made FFI wrapper.
Currently, it returns EFI_INVALID_PARAMETER, meaning that one of the parameters is NULL, for some reason.