Is it a pretty way for exit_boot_services()?

[julic20s]

The current solution is very convenient, but I think some improvements could be added:

1. The memory type of allocation for memory map in exit_boot_services() shouldn't be fixed at MemoryType::LOADER_DATA. Sometime I want to place it into a memory region which has a custom MemoryType.
2. Return (.., MemoryMap<'static>) by value can not keep the lifetime in 'static. For example:

let (_, memory_map) = system_table.exit_boot_services();
let iter: MemoryMapIter<'static> = memory_map.entries(); // error, the lifetime was changed. 

Because the type of memory_map will be MemoryMap<'_> but not MemoryMap<'static>. This causes the lifetime of the buffer to shrink. In fact, the lifetime of memory map buffer != the lifetime of MemoryMap.
MemoryMap::entries() returns a iterator with wrong lifetime.
Currently I have to use a static mutable variable to save the memory map returned by exit_boot_services() to keep the lifetime.

It's a related issue: #523

[julic20s]

Change exit_boot_services() into exit_boot_services(memory_type: MemoryType) to help user to change the memory type of memory map.
But I'm not sure how to fix the second problem. Remove the lifetime of MemoryMap may not a good way.
Use buffer pointer instead of its reference in MemoryMapIter will solve the second problem.

[julic20s]

I will take a pull request.