Release 1.6 (2013-09-10)

In addition to the usual bug fixes, this release has several new features:

The command nix-build --run-env has been renamed to nix-shell.
nix-shell now sources $stdenv/setup inside the interactive shell, rather than in a parent shell. This ensures that shell functions defined by stdenv can be used in the interactive shell.
nix-shell has a new flag --pure to clear the environment, so you get an environment that more closely corresponds to the “real” Nix build.
nix-shell now sets the shell prompt (PS1) to ensure that Nix shells are distinguishable from your regular shells.
nix-env no longer requires a * argument to match all packages, so nix-env -qa is equivalent to nix-env -qa '*'.
nix-env -i has a new flag --remove-all (-r) to remove all previous packages from the profile. This makes it easier to do declarative package management similar to NixOS’s environment.systemPackages. For instance, if you have a specification my-packages.nix like this:
```
with import <nixpkgs> {};
[ thunderbird
  geeqie
  ...
]
```
then after any change to this file, you can run:
```
$ nix-env -f my-packages.nix -ir
```
to update your profile to match the specification.
The ‘with’ language construct is now more lazy. It only evaluates its argument if a variable might actually refer to an attribute in the argument. For instance, this now works:
```
let
  pkgs = with pkgs; { foo = "old"; bar = foo; } // overrides;
  overrides = { foo = "new"; };
in pkgs.bar
```
This evaluates to "new", while previously it gave an “infinite recursion” error.
Nix now has proper integer arithmetic operators. For instance, you can write x + y instead of builtins.add x y, or x < y instead of builtins.lessThan x y. The comparison operators also work on strings.
On 64-bit systems, Nix integers are now 64 bits rather than 32 bits.
When using the Nix daemon, the nix-daemon worker process now runs on the same CPU as the client, on systems that support setting CPU affinity. This gives a significant speedup on some systems.
If a stack overflow occurs in the Nix evaluator, you now get a proper error message (rather than “Segmentation fault”) on some systems.
In addition to directories, you can now bind-mount regular files in chroots through the (now misnamed) option build-chroot-dirs.

This release has contributions from Domen Kožar, Eelco Dolstra, Florian Friesdorf, Gergely Risko, Ivan Kozik, Ludovic Courtès and Shea Levy.