Add Ruby to Rust example

ruby-to-rust/src/lib.rs

@@ -0,0 +1,4 @@
+#[no_mangle]
+pub extern fn double_input(input: i32) -> i32 {
+    input * 2
+}

ruby-to-rust/src/main.rb

@@ -0,0 +1,11 @@
+require 'ffi'
+
+module Hello
+  extend FFI::Library
+  ffi_lib 'target/libdouble_input.so'
+  attach_function :double_input, [ :int ], :int
+end
+
+input = 4
+output = Hello.double_input(input)
+puts "#{input} * 2 = #{output}"