Add Swift to Rust example.

swift-to-rust/Makefile

@@ -0,0 +1,15 @@
+ifeq ($(shell uname),Darwin)
+    EXT := dylib
+else
+    EXT := so
+endif
+
+all: target/debug/libdouble_input.$(EXT)
+	swiftc src/main.swift -import-objc-header src/double.h -L./target/debug -ldouble_input -o double
+	LD_LIBRARY_PATH=./target/debug ./double
+
+target/debug/libdouble_input.$(EXT): src/lib.rs Cargo.toml
+	cargo build
+
+clean:
+	rm -rf target double