This fixes regressions at levels >= 1 which were failing

This fixes regressions at levels >= 1 which were failing

* implement zombification and garbage collection of NodeValues
  (but GC not turned on yet)

* implement removal of key nodes from all attribute tables

* audit NodeBuilder and fix memory leaks and improper reference-count
  management.  This is in many places a re-write.  Clearly documented
  invariants on NodeBuilder state.  (Closes Bug 38)

* created a "BackedNodeBuilder" that can be used to construct
  NodeBuilders with a stack-based backing store for a size that's not
  a compile-time constant.

* NodeValues no longer depend on Node for toStream()'ing

* make unit test-building "silent" with --enable-silent-rules

* (Makefile.am, Makefile.builds.in) fix top-level build system so that
  "make regressN" works with unbuilt/out-of-date source trees in the
  expected way.

* (various) code cleanup, documentation, formatting