Fury script files

[propensive]

We should be able to write an executable script file with the header,

#!/usr/bin/env fury

containing a lightweight version of a Fury build file, following by the terminator, ##, and a Scala source file. Executing such a file would take its hash, check whether a cached compilation of the file already exists, and run it if so. If not, it would be compiled into a cache directory, its main class would be detected, and it would be executed with Java. There's a possibility for it to run as a daemon, but it's not clear how best to wire this up yet.