Re-export all types from the crate root and remove the playlist sub-module

There's not much else in this crate and having it behind another module
decreases visibility.

tests/lib.rs

@@ -1,6 +1,5 @@
 #![allow(unused_variables, unused_imports, dead_code)]
 
-use m3u8_rs::playlist::*;
 use m3u8_rs::*;
 use nom::AsBytes;
 use std::collections::HashMap;