Create update_visitor for modifying trees

Create update_visitor for modifying trees

* staptree.h (update_visitor): A new visitor to make it easier to
rewrite parts of a probe or function without making a full copy.
* staptree.cxx (update_visitor::*): Each child is recursed with a
require() call, and then the parent returns itself with provide().
* staptree.h (deep_copy_visitor): Inherit from update_visitor to get
the recursive descent while updating nodes.
* staptree.cxx (deep_copy_visitor::*): Use the implicit copy
constructors to copy all fields, then defer to update_visitor for the
recursion.  Referents are still cleared from the copies of symbols and
function calls.