Get rid of most uses of ValueRef

Get rid of most uses of ValueRef

Instead pass a const Value & into the functions

With our new inheritance structure, we can get rid of ValueRef
and instead simply pass a pointer to a Value again. Pointers to
Values are safe to use again now, as they are now guaranteed to
be in a place where the GC knows about them.

Change-Id: I44c606fde764db3993b8128fd6fb781d3a298e53
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>