Accommodate the recent changes in the Nightly Rust compiler

- The `macro_reexport` feature was removed and superseded by
  `use_extern_macros`. <rust-lang/rust#29638>
- Switch to the version of `stdsimd` provided directly by the toolchain
  rather than one distributed via crates.io or github.io
- `never_type` is unstable again
  <rust-lang/rust#35121>
- `cfg_target_feature` is stable since 1.27.0
- `target_feature` is stable since 1.27.0

Author: yvt

src/lib.rs

@@ -28,7 +28,7 @@
 //
 #![cfg_attr(test, feature(test))]
 #![cfg_attr(not(debug_assertions), feature(slice_get_slice))]
-#![feature(platform_intrinsics, cfg_target_feature)]
+#![feature(platform_intrinsics)]
 
 extern crate num_complex;
 extern crate num_iter;