Support the Accelerate BLAS on macOS. (#325)

* Add the accelerate feature.

* Ffi tweaks.

candle-core/examples/basics.rs

@@ -1,6 +1,9 @@
 #[cfg(feature = "mkl")]
 extern crate intel_mkl_src;
 
+#[cfg(feature = "accelerate")]
+extern crate accelerate_src;
+
 use anyhow::Result;
 use candle_core::{Device, Tensor};