ReadHeader reads the GPT header and partition entries.

ENTRY_SIZE is the size of the Entry struct.

HEADER_SIZE is the size of the Header struct.

"EFI PART".

NumEntries is the number of entries in the GPT.

HeaderReader is an interface for reading GPT headers.

Entry is a byte slice representing the entry.h C header.

Header is a byte slice representing the header.h C header.