Function parse_entry_at 

pub fn parse_entry_at(bytes: &[u8], offset: usize) -> Option<PackEntry>

Parse one ENTRY_BYTES-sized table entry starting at an arbitrary byte offset. This is the sector-local primitive behind parse_entry: a caller holding a single 2048-byte header sector (not the whole pack) can parse the entries it contains directly, and stitch the one entry that straddles into the next sector through a small ENTRY_BYTES buffer (see entry_location). None when the slice is too short.