Add update_visitor::replace

Add update_visitor::replace

I noticed that most uses of update_visitor::require() were simply
writing the value back to the same place, i.e. foo = require(foo).  The
new replace() method just encapsulates that paradigm, so we don't have
the duplication between the LHS and RHS.

* staptree.h (update_visitor::replace): New.
* elaborate.cxx, staptree.cxx, tapset-mark.cxx, tapset-perfmon.cxx,
  tapset-procfs.cxx, tapset-utrace.cxx, tapsets.cxx: Update all require
  calls that are simply updating the value in-place.