FEAT: Remove odds dependency in arrayvec

Copy the encode_utf8 function from odds. std encode_utf8 requires Rust
1.15 and has a different signature, this one seems to fit us better.

src/array_string.rs

@@ -12,7 +12,7 @@ use std::slice;
 use array::{Array, ArrayExt};
 use array::Index;
 use CapacityError;
-use odds::char::encode_utf8;
+use char::encode_utf8;
 
 #[cfg(feature="serde-1")]
 use serde::{Serialize, Deserialize, Serializer, Deserializer};