* Upgrades the nixpkgs pin to 21.11.
* Removes nix code, sources, and attributes related to haskell.nix and
the code under pkg/hs.
* Continuous integration no longer builds (or packages/releases)
urbit-king (haskell).
* To continue to developing any haskell code under pkg/hs it's left as
a user exercise uto install and configure GHC/Stack.
As a consequence running nix-shell should be considerably more
performant and requires downloading or building less dependencies.
This also removes nixcrpkgs and OSX cross compilation in favour of
compiling on the target. x86_64/musl targets are still supported
on Linux.
All sources are now managed via niv (see nix/sources.json) and Haskell
package sets are provided/organised via IOHK's haskell.nix.
Some effort has been made to expose similar top-level attributes for
development, but in some cases there have been changes. Please see
the comments in the top-level default.nix and ci.nix files for usage.
