refactor: move simple8b encoder/decode

src/encoders/mod.rs

@@ -0,0 +1 @@
+mod simple8b;

src/encoders/simple8b/mod.rs

@@ -205,3 +205,6 @@ pub fn decode(v: u64, dst: &mut [u64]) -> usize {
         _ => 0,
     }
 }
+
+#[cfg(test)]
+mod tests;