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/sharedvector.rs

@@ -459,6 +459,7 @@ fn invalid_capacity_test() {
     let _: SharedVector<u8> = SharedVector::with_capacity(usize::MAX / 2 - 1000);
 }
 
+#[cfg(not(target_arch = "wasm32"))]
 pub(crate) mod ffi {
     use super::*;