gvalue: New g_value_init_from_instance

Used for the commonly used case (in signal emission) where we
initialize and set a GValue for an instance

Includes a fast-path for GObject

Overall makes it 6 times faster than the previous combination
of g_value_init + g_value_set_instance

Makes signal emission around 10% faster

https://bugzilla.gnome.org/show_bug.cgi?id=731950