stdenv: introduce baseHash() to replace stripHash()

stripHash uses a global variable to communicate it's computation
results, but it's not necessary. You can just pipe to stdout in a
subshell. A function mostly behaves like just another command.

baseHash() also introduces a suffix-stripping capability since it's
something the users of the function tend to use.

pkgs/data/fonts/droid/default.nix

@@ -41,7 +41,7 @@ stdenv.mkDerivation rec {
   sourceRoot = "./";
 
   unpackCmd = ''
-    ttfName=$(basename $(stripHash $curSrc; echo $strippedName))
+    ttfName=$(baseHash $curSrc)
     cp $curSrc ./$ttfName
   '';