multiboot2-header: various improvements

[phip1611]

• rename std module to alloc
• multiboot2_header::load should return not only a byte slice (seen in add missing functionality in multiboot2-header (finding the header, getting tags) #136)
• also, the assertion could be transformed into a variant of the error enum

[YtvwlD]

Yeah, I'm not sure about the byte slice. This can be useful to be able to discard the rest of the file.

[phip1611]

It's okay, I can think about it and work on it eventually. This is probably technical debt that wasn't introduced by you