This makes it easier for more complicated packages (e.g. network, which
needs to install a C shared library) to know where to put things without
having to work out the prefix themselves.
Since they'll be incompatible between different Idris2 versions, this
helps protect against importing the wrong thing by mistake. Also, it
means the canonical place for the version number is now the top level
Makefile.
You'll need to delete src/YafflePaths.idr before rebuilding, since it's
now generated slightly differently.
This is so that we can put other build artefacts (e.g. executables) in
properly organised subdirectories of build, e.g. build/bin/chez,
build/bin/js, etc.
