Remove extern "C" functions from wasm module

We don't need these functions and their export. They account for ~20kb in
the optimized .wasm - plus JS glue code.

sixtyfps_runtime/corelib/string.rs

@@ -316,6 +316,7 @@ fn simple_test() {
     );
 }
 
+#[cfg(not(target_arch = "wasm32"))]
 pub(crate) mod ffi {
     use super::*;