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.

examples/simple.rs

@@ -1,4 +1,4 @@
-use m3u8_rs::playlist::Playlist;
+use m3u8_rs::Playlist;
 use std::io::Read;
 
 fn main() {

examples/with_nom_result.rs

@@ -1,4 +1,4 @@
-use m3u8_rs::playlist::Playlist;
+use m3u8_rs::Playlist;
 use std::io::Read;
 
 fn main() {

src/lib.rs

@@ -1,4 +1,5 @@
-pub mod playlist;
+mod playlist;
+pub use playlist::*;
 
 #[cfg(feature = "parser")]
 mod parser;

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;