Begin to add tahoe-great-black-swamp as a dependency

It doesn't build.

obelisk/dep/tahoe-great-black-swamp/default.nix

+# DO NOT HAND-EDIT THIS FILE
+import (import ./thunk.nix)
\ No newline at end of file

obelisk/dep/tahoe-great-black-swamp/git.json

+{
+  "url": "https://whetstone.private.storage/privatestorage/tahoe-great-black-swamp.git",
+  "rev": "8258c03db455d820422e9b5fa8da29384a236791",
+  "sha256": "1fq71nz3zr2chkrh0m4c4y19iq54lvvj35m6k080fhvw5g1l84vz",
+  "private": false,
+  "fetchSubmodules": false
+}

obelisk/dep/tahoe-great-black-swamp/thunk.nix

+# DO NOT HAND-EDIT THIS FILE
+let fetch = {url, rev, branch ? null, sha256 ? null, fetchSubmodules ? false, private ? false, ...}:
+  let realUrl = let firstChar = builtins.substring 0 1 url; in
+    if firstChar == "/" then /. + url
+    else if firstChar == "." then ./. + url
+    else url;
+  in if !fetchSubmodules && private then builtins.fetchGit {
+    url = realUrl; inherit rev;
+    ${if branch == null then null else "ref"} = branch;
+  } else (import (builtins.fetchTarball {
+  url = "https://github.com/NixOS/nixpkgs/archive/3aad50c30c826430b0270fcf8264c8c41b005403.tar.gz";
+  sha256 = "0xwqsf08sywd23x0xvw4c4ghq0l28w2ki22h0bdn766i16z9q2gr";
+}) {}).fetchgit {
+    url = realUrl; inherit rev sha256;
+  };
+  json = builtins.fromJSON (builtins.readFile ./git.json);
+in fetch json
\ No newline at end of file

obelisk/frontend/frontend.cabal

@@ -21,6 +21,7 @@ library
                , text
                , bytestring
                , tahoe-chk
+               , tahoe-great-black-swamp
                , req
                , extra
                , reflex-material

obelisk/haskell-overrides.nix

@@ -5,6 +5,15 @@ with pkgs.haskell.lib; {
   # get a nix derivation to build it.
   tahoe-chk = self.callCabal2nix "tahoe-chk" (nix-thunk.thunkSource ./dep/tahoe-chk) {};
 
+  # Similarly, get a package implementing Great Black Swamp.
+  tahoe-great-black-swamp = self.callCabal2nix "tahoe-great-black-swamp" (nix-thunk.thunkSource ./dep/tahoe-great-black-swamp) {};
+
+  statvfs = self.callHackageDirect {
+    pkg = "statvfs";
+    ver = "0.2";
+    sha256 = "sha256-Q8VeTrxrcaZWRsfvYKNNmCnhb+SXWByBATiFB6+mUOM=";
+  } {};
+
   # We also ended up needing an override of the base32 library, which we obtain from Hackage.
   base32 = self.callHackageDirect {
     pkg = "base32";