Add From() binding for UPSERTs

[0x6e6562]

e72de39 added the Apply(...ColumnBindings) for generating UPSERTs using ColumnBindings. This is probably messy, since you need to use the To() method to create the binding, which accepts a destination pointer. Whilst this works, I think it would be clearer to offer an explicit non-pointer method such as From(). If we were to do this, we'd probably need to create a specialized type of ColumnBinding to avoid overloading the legal arguments for To() and From().