Remove usage of memoffset::offset_of

[syberant]

Hi, I may or may not have used your crate but I'd like to say a quick thank you for it anyway!
I'm going down the list of reverse dependencies on memoffset.

This PR originally aimed to remove the memoffset crate from your dependencies.
core::mem::offset_of was stabilised in rustc 1.77 which I believe is at or below your MSRV.

The memoffset crate 0.9.1 says that

If you're using a rustc version greater or equal to 1.77,
this crate's offset_of!() macro simply forwards to core::mem::offset_of!().

---

A singular use of memoffset::span_of! prevents removing the dependency, if you're willing to hardcode it we could.

    pub(crate) fn signature(&self) -> [u8; 4] {
        self.data[span_of!(RecordHeader, signature)]
            .try_into()
            .unwrap()
    }

where

#[repr(C, packed)]
pub(crate) struct RecordHeader {
    signature: [u8; 4],
    update_sequence_offset: u16,
    update_sequence_count: u16,
    logfile_sequence_number: u64,
}