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" ];
+}
+```

snippets/nixctor/ctor.nix

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+# ctor.nix: simple buildsystem written in Nix
+# It can be adapted for any language, just substitute the default arguments
+args @
+{ pkgs ? import <nixpkgs> { }
+, lib ? pkgs.lib
+, stdenv ? pkgs.stdenv
+
+# Name of the compiler to use when compiling source code
+, compiler ? "cc"
+# Flags passed to the compiler during compilation stages.
+, cflags ? "-O2 -c"
+# Flags passed to the compiler at link time
+, ldflags ? ""
+# Flag passed to the compiler to specify the output file
+, outFlag ? "-o"
+# Source code files to compile
+, sourceFiles ? [ "main.c" ]
+# Suffix of source code files
+, suffix ? ".c"
+# Name of the binary file to produce
+, programName ? "main"
+
+, ...
+}:
+
+with builtins;
+let
+  newline = "
+";
+  run = command: ''
+    printf '%s\n' "${command}"
+    ${command}
+  '';
+  objify = sourceFile: replaceStrings [ suffix ] [ ".o" ] sourceFile;
+  compile = sourceFile:
+    "${compiler} ${cflags} ${outFlag} ${objify sourceFile} ${sourceFile}";
+  link = objs:
+    "${compiler} ${outFlag} ${programName} ${concatStringsSep " " objs} ${ldflags}";
+in
+stdenv.mkDerivation ({
+  buildPhase = ''
+    runHook preBuild
+    ${concatStringsSep newline (map
+      (source: run (compile source)) sourceFiles)}
+    ${run "${link (map objify sourceFiles)}"}
+    runHook postBuild
+  '';
+
+  installPhase = ''
+    runHook preInstall
+    install -Dm755 ${programName} $out/bin/${programName}
+    runHook postInstall
+  '';
+} // args)

snippets/nixctor/default.nix

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+{ constructProgram ? import ./ctor.nix }:
+constructProgram {
+  pname = "hello-world";
+  version = "1.0.0";
+
+  src = ./.; # use the current directory
+
+  sourceFiles = [ "main.c" "lib.c" ];
+}

snippets/nixctor/lib.c

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+#include <stdio.h>
+
+void
+libfunc(void)
+{
+	puts("hello, world!");
+}

snippets/nixctor/main.c

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+void libfunc(void);
+
+int
+main(int argc, char *argv[])
+{
+	libfunc();
+}