RFC: Re-design SystemTable/BootServices/RuntimeServices

[medhefgo]

Right now, accessing any of the fundamental UEFI tables is very annoying and sometimes leads to annoying lifetime issues.

Apps either have to resort to passing a st/bs/rt reference around everywhere or have to resort to the un-ergonomic (because it's unsafe and doesn't even return a proper reference) system_table() from uefi-services. The former isn't even viable if implementing an API/protocol with fixed function signatures.

There are two things to note:

• BootServices/RuntimeServices are really just a list of global functions. The only reason they are provided through SystemTable is because UEFI isn't a hosted environment with dynamic linking.
• No functions in them take a this pointer.

I propose the API to be changed such that:

• The application's entry-point is changed to a simple fn main() -> Result signature. The #[entry] (which I would prefer renamed to efi_main) provides the real entry-point internally, stores the SystemTable and image handle somewhere, and calls into the marked main function. On return, it does the right thing(tm) with the Result (print out a message and return status code).
• All functions in BootServices/RuntimeServices are changed to associated functions that access the global table pointer internally. As they don't take a this pointer, they are just global functions as is basically done in EDK2 (well, they provide a global pointer to the table struct, same difference). It's more ergonomic and will eliminate some annoying lifetime issues. It also obviates the need for some structs like ScopedProtocol to keep a ref to the table.
• To protect against use after exiting boot services, the BootServices table pointer can be replaced by the exit handler with a None or replaced with a custom implementation that panics if actually used.
• Access to stdin/stdout/stderr should be provided via associated functions from the Input/Output types (and panic after boot services exit).
• An associated function would be provided to get direct access to the tables if needed.
• This would also remove the need for a View on SystemTable: any members that go away should be Options and set to None once boot services are exited by the exit handler. (In fact, the UEFI spec recommends doing so.)