dag: remove unnecessary FileNode.Changed() method

nodebase.Changed is perfectly fine.
dag: factor out some duplicate code between FileNode and StubNode.
dag: add ActionNode, for incremental testing/linting/etc
update to latest version of golex (and friends)

upstream commit IDs:
  github.com/cznic/golex: cb1e42de2bc56c7d1bb4522812018d3eb7062c1b
  github.com/cznic/lex: a201ad5e3dcc0190f5c71dea083de132f97c69d1

closes issue #2
really update to the latest golex (missed 'lexer' package)

upstream commit IDs:
  github.com/cznic/lexer: 6e3605e3bb502046f633c8f546d6390dd1ce9ed4