Move enum-print to elf/

In preparation for pretty-printing ELF enums
Move to_hex from dwarf/internal.hh to elf/to_hex.hh

In preparation for pretty-printing ELF enums
elf: Full set of bitmask operators for bitmask types
elf: to_string functions for all ELF enum types

To make this possible, this also adds support for mask types to
enum-print.py, along with a few other options.
elf: Define values of special section indices

And a pretty-printer for section indices.
examples: Dump ELF sections and symbol tables