Add Rust to C example

rust-to-c/src/double.c

@@ -0,0 +1,3 @@
+int double_input(int input) {
+    return input * 2;
+}

rust-to-c/src/main.rs

@@ -0,0 +1,11 @@
+extern crate libc;
+
+extern {
+    fn double_input(input: libc::c_int) -> libc::c_int;
+}
+
+fn main() {
+    let input = 4;
+    let output = unsafe { double_input(input) };
+    println!("{} * 2 = {}", input, output);
+}