simplify checksum/preview stuff using fullRunInputTask

(I don't think fullRunInputTask existed back when I first wrote this
stuff)
relocate ChecksumsAndPreviews to build.sbt

yay sbt 0.13!
convert project/Classycle.scala to .sbt format

and start putting custom tasks in tasks.sbt, separate from build.sbt,
just so build.sbt doesn't get enormous
reimplement project/Dump.scala a better way
move JFlex stuff to subdirectory

just tidying up
consolidate source/resource generators into generators.sbt
cleanup/reorg in build.sbt
pile project/Extensions.scala into tasks.sbt
combine two files that didn't really need to be separate

and having two files named "build.sbt" was a bit confusing
fold project/Scaladoc.scala into build.sbt
reformat
minor code improvement
sbt build: split Testing into two parts

the parts didn't really have anything to do with each other, so better
to keep them separate
rename testChecksums task to just ts

to match tr, tc, tm, te
Merge branch 'headless' into sbt-0.13.1