There is no public constructor for MemoryMapOwned, as the construction of it isquite complicated and therefore hidden behind BootServices::memory_map() respectivelySystemTable::exit_boot_services.The real-world use case for the public constructors for the other types is whenparsing the UEFI memory map from kernels after a bootloader, such as GRUB or Limine,passed the UEFI memory map via the corresponding boot information.

Simplify MemoryMapOwned::from_initialized_mem to simplify and streamline theconstruction in unit tests.

Add a test that imitates how a public API user would utilize our abstractions toparse a chunk of memory as UEFI memory map.