Add a simplification rule for tagged pointers.

The rule turns any value type assigment hypothesis for a tagged pointer
with tag 0 into a value type assigment hypothesis for an owned pointer.
This allows implementing the [untag] function using [tag] in the tagged
pointer example (examples/tagged_ptr.c).
2 jobs for ci/tagged_ptr_rule in 17 minutes and 52 seconds (queued for 2 seconds)