Add build mode to compute fixed-output derivation hashes

For example,

  $ nix-build --hash -A nix-repl.src

will build the fixed-output derivation nix-repl.src (a fetchFromGitHub
call), but instead of *verifying* the hash given in the Nix
expression, it prints out the resulting hash, and then moves the
result to its content-addressed location in the Nix store. E.g

  build produced path ‘/nix/store/504a4k6zi69dq0yjc0bm12pa65bccxam-nix-repl-8a2f5f0607540ffe56b56d52db544373e1efb980-src’ with sha256 hash ‘0cjablz01i0g9smnavhf86imwx1f9mnh5flax75i615ml71gsr88’

The goal of this is to make all nix-prefetch-* scripts unnecessary: we
can just let Nix run the real thing (i.e., the corresponding fetch*
derivation).

Another example:

  $ nix-build --hash -E 'with import <nixpkgs> {}; fetchgit { url = "https://github.com/NixOS/nix.git"; sha256 = "ffffffffffffffffffffffffffffffffffffffffffffffffffff"; }'
  ...
  git revision is 9e7c1a4bbd
  ...
  build produced path ‘/nix/store/gmsnh9i7x4mb7pyd2ns7n3c9l90jfsi1-nix’ with sha256 hash ‘1188xb621diw89n25rifqg9lxnzpz7nj5bfh4i1y3dnis0dmc0zp’

(Having to specify a fake sha256 hash is a bit annoying...)

src/nix-store/nix-store.cc

@@ -117,6 +117,7 @@ static void opRealise(Strings opFlags, Strings opArgs)
         if (i == "--dry-run") dryRun = true;
         else if (i == "--repair") buildMode = bmRepair;
         else if (i == "--check") buildMode = bmCheck;
+        else if (i == "--hash") buildMode = bmHash;
         else if (i == "--ignore-unknown") ignoreUnknown = true;
         else throw UsageError(format("unknown flag ‘%1%’") % i);