Move base16_encode() to its own header file
Introduce symboldb_options::rpm_cache() and symboldb_options::usage_error
Remove the SQL implementation of the ELF closure computation
Add base16_decode
Add file_cache::digests()
Add database::referenced_package_digests()
Add --show-stale-cached-rpms
Add column symboldb.file.length
Add column symboldb.elf_image.e_type
Fix indentation
Add TODO item: libpq binary interface
rpm_load: refactor
Change type of rpm_file_entry::contents

This is for consistency with the source/sink abstractions.
Add columns symboldb.file.digest, symboldb.file.contents
Remove TODO item