For DWARF 4, we currently allow range list entries to use a pair of addresses. However, the DWARF 4 standard (section 2.17.3) only talks about address offsets. We should check if our handling is correct. Maybe we should use read::RawRngListEntry::OffsetPair instead for DWARF 4.

Related to this, read::Unit::low_pc has type u64. This may be better as Option<u64>, and then an offset pair without a base address can be detected as an error.