move dmath, dsafe and nixctor to snippets/ directory

snippets/nixctor/README.md

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+## ctor.nix - C/C++/D buildsystem written in Nixlang
+
+Here is a buildsystem I made that incrementally compiles a program and
+then links it together at the end. The file default.nix shows an
+example on how to use this code in a real derivation, it's as simple
+as just adding your source files into the `sourceFiles` array and
+ctor.nix will generate a build phase and an install phase for you. It
+just calls `stdenv.mkDerivation` so you'll have to supply the required
+arguments to that too.
+
+To test:
+
+    nix-build
+
+To use a different compiler or flags, you can use the `compiler` and
+`cflags` arguments:
+
+```nix
+# Example for a D program
+constructProgram {
+  pname = "hello-world";
+  version = "1.0.0";
+  
+  src = fetchFromWhatever {};
+
+  compiler = "ldc2";
+  cflags = "-O -wi";
+  sourceFiles = [ "main.d" "lib.d" ];
+}
+```