Support simd_json (#105)

This PR adds support for the simd-json library whenever decoding or encoding JSON responses. This may be enabled independently of serenity and twilight support for SIMD acceleration.

Co-authored-by: Kyle Simpson <kyleandrew.simpson@gmail.com>

src/error.rs

@@ -2,8 +2,12 @@
 
 #[cfg(feature = "serenity")]
 use futures::channel::mpsc::TrySendError;
+#[cfg(not(feature = "simd-json"))]
+pub use serde_json::Error as JsonError;
 #[cfg(feature = "serenity")]
 use serenity::gateway::InterMessage;
+#[cfg(feature = "simd-json")]
+pub use simd_json::Error as JsonError;
 #[cfg(feature = "gateway")]
 use std::{error::Error, fmt};
 #[cfg(feature = "twilight")]