Always show actual literals in bytecode printer

Currently the keyword "literal" is used inconsistently across the bytecode
instructions. For example, push_const uses it, but set_const doesn't.

Always use "literal" to indicate the index to literals. And this enables
Rubinius' built-in bytecode printer to show the actual literals referenced by
the index.

This would really ease my life when hacking on Rubinus, hopefully others
starting to hack on it.
Merge pull request #2144 from ryoqun/print-bytecode-literals

Always show actual literals in bytecode printer