Prevent use-after-free with vectors

Awhile back slices switched to being raw views into wasm memory, but this
doens't work if we free the underlying memory unconditionally! Moving around a
`Vec` is already moving a lot of data, so let's copy it onto the JS heap instead
of leaving it in the wasm heap.

crates/cli-support/src/js/js2rust.rs

@@ -256,7 +256,7 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
                 const mem = getUint32Memory();\n\
                 const ptr = mem[retptr / 4];\n\
                 const len = mem[retptr / 4 + 1];\n\
-                const realRet = {}(ptr, len);\n\
+                const realRet = {}(ptr, len).slice();\n\
                 wasm.__wbindgen_free(ptr, len * {});\n\
                 return realRet;\n\
             ", f, ty.size());