Implement wasm_config_new and wasm_engine_new_with_config (#901)

* Implement wasm_config_new and wasm_engine_new_with_config

crates/c-api/examples/fib-wasm.c

@@ -0,0 +1,13 @@
+// Compile with:
+//   clang --target=wasm32 fib-wasm.c -o fib-wasm.wasm -g \
+//     -Wl,--no-entry,--export=fib -nostdlib -fdebug-prefix-map=$PWD=.
+
+int fib(int n) {
+  int i, t, a = 0, b = 1;
+  for (i = 0; i < n; i++) {
+    t = a;
+    a = b;
+    b += t;
+  }
+  return b;
+}