Introduces new builders, mainly `SectionTable` and `StringTable`, and a
final `build_elf_image` to merge everything into a single memory image.
Each of the builders are fully detached from one another, although
StringTable provides an extra API to remove steps when using it with a
SectionTable.
This automates the part of figuring out and properly writing offsets to
headers, and making sure all required data is properly copied and
referenced in the final image.
