Fix procedural macros

[necauqua]

Currently procedural macros do not work outside of uefi crate, because they are referencing crate-local items (Guid, Protocol etc.) and when invoked from a crate other than 'uefi' those items cannot be found (well because the other crate does not have them).

This is a known (as I learned) procedural macro issue and the latest best workaround is what I implemented here - allow 'uefi' to reference itself by ::uefi and then use ::uefi in those macros.

As an additional bonus (I can revert that if you have objections but it's neat and does not break anything) I made the 'entry' macro always make the exported entry function have the name 'efi_main', which is required by the rust efi target I think.

Anyway, with that change, users can name their entry function whatever they want (right now that leads to cryptic errors iirc), and the fact that this is an entry function is clearly marked by the #[entry] attribute above it.

[GabrielMajeri]

Thank you, the changes look great!

I made the 'entry' macro always make the exported entry function have the name 'efi_main', which is required by the rust efi target I think

Could you also update the docs to mention that the main function can be named in any way, but will be renamed to efi_main by the proc macro?

[necauqua]

I've also added the 'extern crate rlibc' thing to said doc, because that one took me an extra minute at some point :)

[GabrielMajeri]

Thanks for the contribution!