Use traits instead of macros for shift intrinsics

This is an attempt to tidy up the definition of intrinsics by making them more
rust-like at the definition site and using traits instead of macros for the
definition. Additionally the helper macro, `intrinsics!`, now fills in a
definition for #[cfg]'d off intrinsics when compiling with C code

library/compiler-builtins/src/lib.rs

@@ -99,6 +99,9 @@ fn sconv(i: i128) -> U64x2 {
 #[cfg(test)]
 extern crate core;
 
+#[macro_use]
+mod macros;
+
 pub mod int;
 pub mod float;