syntax: back to -> for return type, no more =>

example/shared_library/mathtest.zig

@@ -1,10 +1,10 @@
 #version("2.0.0")
 export library "mathtest";
 
-export fn add(a: i32, b: i32) i32 => {
+export fn add(a: i32, b: i32) -> i32 {
     a + b
 }
 
-export fn hang() unreachable => {
+export fn hang() -> unreachable {
     while (true) { }
 }