create the windows io mod and link printf (#144)

lib/emscripten/src/io/unix.rs

@@ -0,0 +1,17 @@
+use libc::printf as _printf;
+
+use wasmer_runtime_core::vm::Ctx;
+
+/// putchar
+pub fn putchar(chr: i32, ctx: &mut Ctx) {
+    unsafe { libc::putchar(chr) };
+}
+
+/// printf
+pub fn printf(memory_offset: i32, extra: i32, ctx: &mut Ctx) -> i32 {
+    debug!("emscripten::printf {}, {}", memory_offset, extra);
+    unsafe {
+        let addr = emscripten_memory_pointer!(ctx.memory(0), memory_offset) as _;
+        _printf(addr, extra)
+    }
+}