database: Switch to std::tuple
cxxll::dwarf::error: New struct
cxxll::elf_image::dwarf_cu_range: Rewrite error handling
SQL schema: Add missing index symboldb.dwarf_compilation_unit (contents_id)

It is required for efficient database expiration.
SQL schema: Add table symboldb.dwarf_error
database::add_dwarf: Add errors to table symboldb.dwarf_error
Integration test: Check symboldb.dwarf_error table contents
cxxll::elf_image::dwarf_cu_range::die_offset(): New member function
database::add_dwarf(): Perform UTF-8 normalization if necessary