Merge staging into closure-size

The most complex problems were from dealing with switches reverted in
the meantime (gcc5, gmp6, ncurses6).
It's likely that darwin is (still) broken nontrivially.

pkgs/development/libraries/icu/default.nix

@@ -2,7 +2,7 @@
 
 let
   pname = "icu4c";
-  version = "55.1";
+  version = "56.1";
 in
 stdenv.mkDerivation {
   name = pname + "-" + version;
@@ -10,7 +10,7 @@ stdenv.mkDerivation {
   src = fetchurl {
     url = "http://download.icu-project.org/files/${pname}/${version}/${pname}-"
       + (stdenv.lib.replaceChars ["."] ["_"] version) + "-src.tgz";
-    sha256 = "0ys5f5spizg45qlaa31j2lhgry0jka2gfha527n4ndfxxz5j4sz1";
+    sha256 = "05j86714qaj0lvhvyr2s1xncw6sk0h2dcghb3iiwykbkbh8fjr1s";
   };
 
   outputs = [ "dev" "out" ];