Add `Protocol.pack`.
Add `Protocol.size` and `Protocol.alignment`.
Add set method to Protocol.
Add `String#pointer` to get raw address of String's buffer.
Undefine some methods of String instance whose pointer is exported.
Implement `Protocol.to_s`.
Implement Protocol#to_s.
Add `Pointer#initialize` and `Pointer#initialize_copy`.
Implement `Pointer#to_i`.
Implement `Pointer#+` and `Pointer#-`.
Implement `Pointer#read` and `Pointer#write`.
Refinement. Use mrb_uefi_pointer_raw_value.