cxxll::elf_image: Make interp extraction more robust

This still extracts garbage for separate debuginfo files, unfortunately.
database::java_class(): Reformat
cxxll::java_class::reference: New class

Also adds member function cxxll::java_class::references(),
which extracts field and method references.
SQL schema: Add table symboldb.java_reference
database::add_java_class(): Load table symboldb.java_reference