Keep the Store alive until its StoreContext is no longer used. (#206)

As noted in #200, this is required as otherwise it could theoretically happen that the `Store` handle is deleted (with `wasmtime_store_delete`) in the GC finalizer thread even when at the same time a native call using the `StoreContext` handle is still executing in another thread, in case the `Store` object is no longer reachable.

For native methods taking a handle parameter that is passed as `SafeHandle`, this is not required as the `SafeHandle` is already kept alive during the call.
An exception is if you use `SafeHandle.DangerousGetHandle()` to retrieve the handle as pointer value and pass it; in that case you must also keep the `SafeHandle` alive.

This commit also fixes an instance of the above noted issue in `Engine.IncrementEpoch()`, where `wasmtime_engine_increment_epoch` was declared as taking an `IntPtr` handle, and `Engine.IncrementEpoch()` used `SafeHandle.DangerousGetHandle()` without keeping the SafeHandle alive (this appears to have been introduced with #118).

Author: kpreisser

src/Engine.cs

@@ -41,7 +41,7 @@ public void Dispose()
         /// </summary>
         public void IncrementEpoch()
         {
-            Native.wasmtime_engine_increment_epoch(handle.DangerousGetHandle());
+            Native.wasmtime_engine_increment_epoch(handle);
         }
 
         internal Handle NativeHandle
@@ -84,7 +84,7 @@ private static class Native
             public static extern void wasm_engine_delete(IntPtr engine);
 
             [DllImport(LibraryName)]
-            public static extern void wasmtime_engine_increment_epoch(IntPtr engine);
+            public static extern void wasmtime_engine_increment_epoch(Handle engine);
         }
 
         private readonly Handle handle;