Release X.Y (202?-??-??)

Various nix commands can now read expressions from stdin with --file -.
nix store make-content-addressable has been renamed to nix store make-content-addressed.
New experimental builtin function builtins.fetchClosure that copies a closure from a binary cache at evaluation time and rewrites it to content-addressed form (if it isn't already). Like builtins.storePath, this allows importing pre-built store paths; the difference is that it doesn't require the user to configure binary caches and trusted public keys.

This function is only available if you enable the experimental feature fetch-closure.
New experimental feature: impure derivations. These are derivations that can produce a different result every time they're built. Here is an example:
```
stdenv.mkDerivation {
  name = "impure";
  __impure = true; # marks this derivation as impure
  buildCommand = "date > $out";
}
```
Running nix build twice on this expression will build the derivation twice, producing two different content-addressed store paths. Like fixed-output derivations, impure derivations have access to the network. Only fixed-output derivations and impure derivations can depend on an impure derivation.