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/layout.rs

@@ -778,6 +778,7 @@ pub fn solve_path_layout(data: &PathLayoutData) {
     }
 }
 
+#[cfg(not(target_arch = "wasm32"))]
 pub(crate) mod ffi {
     #![allow(unsafe_code)]