Add flake evaluation cache

This exploits the hermetic nature of flake evaluation to speed up
repeated evaluations of a flake output attribute.

For example (doing 'nix build' on an already present package):

  $ time nix build nixpkgs:firefox

  real    0m1.497s
  user    0m1.160s
  sys     0m0.139s

  $ time nix build nixpkgs:firefox

  real    0m0.052s
  user    0m0.038s
  sys     0m0.007s

The cache is ~/.cache/nix/eval-cache-v1.sqlite, which has entries like

  INSERT INTO Attributes VALUES(
    X'92a907d4efe933af2a46959b082cdff176aa5bfeb47a98fabd234809a67ab195',
    'packages.firefox',
    1,
    '/nix/store/pbalzf8x19hckr8cwdv62rd6g0lqgc38-firefox-67.0.drv /nix/store/g6q0gx0v6xvdnizp8lrcw7c4gdkzana0-firefox-67.0 out');

where the hash 92a9... is a fingerprint over the flake store path and
the contents of the lockfile. Because flakes are evaluated in pure
mode, this uniquely identifies the evaluation result.

src/libexpr/flake/eval-cache.hh

@@ -0,0 +1,39 @@
+#pragma once
+
+#include "sync.hh"
+#include "flake.hh"
+
+namespace nix { struct SQLite; struct SQLiteStmt; }
+
+namespace nix::flake {
+
+class EvalCache
+{
+    struct State;
+
+    std::unique_ptr<Sync<State>> _state;
+
+    EvalCache();
+
+public:
+
+    struct Derivation
+    {
+        Path drvPath;
+        Path outPath;
+        std::string outputName;
+    };
+
+    void addDerivation(
+        const Fingerprint & fingerprint,
+        const std::string & attrPath,
+        const Derivation & drv);
+
+    std::optional<Derivation> getDerivation(
+        const Fingerprint & fingerprint,
+        const std::string & attrPath);
+
+    static EvalCache & singleton();
+};
+
+}