Enable the multi-value proposal by default (#1667)

This was merged into the wasm spec upstream in WebAssembly/spec#1145, so
let's follow the spec and enable it by default here as well!

examples/multi.c

@@ -61,10 +61,7 @@ wasm_trap_t* closure_callback(
 int main(int argc, const char* argv[]) {
   // Initialize.
   printf("Initializing...\n");
-  wasm_config_t *config = wasm_config_new();
-  assert(config != NULL);
-  wasmtime_config_wasm_multi_value_set(config, true);
-  wasm_engine_t* engine = wasm_engine_new_with_config(config);
+  wasm_engine_t* engine = wasm_engine_new();
   wasm_store_t* store = wasm_store_new(engine);
 
   // Load our input file to parse it next

examples/multi.rs

@@ -11,11 +11,8 @@ use anyhow::{format_err, Result};
 use wasmtime::*;
 
 fn main() -> Result<()> {
-    // Configure our `Store`, but be sure to use a `Config` that enables the
-    // wasm multi-value feature since it's not stable yet.
     println!("Initializing...");
-    let engine = Engine::new(Config::new().wasm_multi_value(true));
-    let store = Store::new(&engine);
+    let store = Store::default();
 
     // Compile.
     println!("Compiling module...");