speed up jar generation

[cvogt]

investigate performance improvements in creating jars via FastPackage ( https://github.com/fommil/sbt-big-project/blob/master/src/main/scala/BigProjectSettings.scala#L473 ).

To avoid an external dependency we could copy the code into cbt.

Related gitter discussion: https://gitter.im/cvogt/cbt?at=5964f2d3bc46472974dbb033

[cvogt]

cc @fommil @darthorimar

[fommil]

I'd really love to have an fs2 wrapper around nio, it would be ideal for writing this sort of thing in a functional (and fast) way. That code is a bit... Java... if you know what I mean.