Fancy build: grand unified build system for all and sundry.

Models constraints of and is able to interoperate with any other build system.
(Ideally.  In practice, something like scons will be impractical; but make,
 cargo, cpan, dub, etc. are fine.)

Closest in design to shake (https://shakebuild.com/).  Primary differences:
 - Build starts instantly.  No need to build your build file.
 - No conflation of build targets with on-disc files.  Represent deployment,
   dependencies from (network) package registry, ...