Move macros.rs to first.rs

This awkwardly has to go first right now, and we don't want more
macros to go into this ambiguously named mod, so let's rename it
to be more literal.

Author: workingjubilee

crates/core_simd/src/macros.rs renamed to crates/core_simd/src/first.rs

@@ -5,7 +5,7 @@
 //! Portable SIMD module.
 
 #[macro_use]
-mod macros;
+mod first;
 #[macro_use]
 mod permute;
 #[macro_use]