Implement support for passing Go functions as custom functions to SQLite.

Fixes #226.

callback.go

@@ -0,0 +1,20 @@
+// Copyright (C) 2014 Yasuhiro Matsumoto <mattn.jp@gmail.com>.
+//
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package sqlite3
+
+/*
+#include <sqlite3-binding.h>
+*/
+import "C"
+
+import "unsafe"
+
+//export callbackTrampoline
+func callbackTrampoline(ctx *C.sqlite3_context, argc int, argv **C.sqlite3_value) {
+	args := (*[1 << 30]*C.sqlite3_value)(unsafe.Pointer(argv))[:argc:argc]
+	fi := (*functionInfo)(unsafe.Pointer(C.sqlite3_user_data(ctx)))
+	fi.Call(ctx, args)
+}