Add formatting for int literal suffixes

[RedDaedalus]

Adds rewrite rules and configuration options for suffixed int literals:

// literal_suffix_style: Join
let x = 0i32;
let y = 0xFFi32;

// literal_suffix_style: Separate
let x = 0_i32;
let y = 0xFF_i32;

// literal_suffix_style: Preserve
let x = 0i32;
let y = 0xFF_i32;

I'm not sure on the naming for these options -- if someone has a better idea I would like to rename it.