NEW IN WAF 2.0.0
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* Reverse the mappping in TaskGen.prec[]
* Sort TaskGen methods besides regular constraints
* Removed ut_fun from waf_unit_test.py
* Force files into the build directory by default

* Remove Node.sig and Node.cache_sig
* Remove the BuildContext.rule decorator
* Remove Task.update_outputs, Task.always_run
* Remove atleast-version, exact-version and max-version from conf.check_cfg
* Remove c_preproc.trimquotes