Change wasm_module_new to use Module::from_binary.

This commit changes the C API function `wasm_module_new` to use the Rust API
`Module::from_binary` which performs verification of the module, as per the C
API spec.

This also introduces a `EngineBuilder` type to the C# API that can be used to
construct an `Engine` with the various Wasmtime configuration options.  This
is required to get the C# tests passing since they use reference types and
multi-value.

Fixes #859.

crates/misc/dotnet/src/Engine.cs

@@ -20,6 +20,17 @@ namespace Wasmtime
             }
         }
 
+        internal Engine(Interop.WasmConfigHandle config)
+        {
+            Handle = Interop.wasm_engine_new_with_config(config);
+            config.SetHandleAsInvalid();
+
+            if (Handle.IsInvalid)
+            {
+                throw new WasmtimeException("Failed to create Wasmtime engine.");
+            }
+        }
+
         /// <summary>
         /// Creates a new Wasmtime <see cref="Store" />.
         /// </summary>