Support writing of one unit at a time #805
Description
Currently the DWARF writing code requires all units to be built up in memory before starting the writing to the sections. When running the simple_convert example on large files, this results in high memory usage. We should change the conversion so that it can convert and write one unit at a time. Writing of cross-unit references will need to be delayed. We'll also need to be able to calculate .debug_str offsets before the string table is completely built.