Added the dbg and format filters

[Kixiron]

• Added the dbg filter which prints the passed value to stdout (similar to the dbg!() macro)
• Added the format filter, which offers various integer formatting routines such as :x and :X

Closes #474

[Keats]

If you can split dbg from format yes

[merkrafter]

What is your opinion on including the formatting as a parameter to the as_str builtin? Also, it seems like only formatting a single value is supported, hence using a name (e.g. "hex", "Octal") instead of a format string (like ":x", ":O") might be the easier to understand interface.