Implement a suggestions mechanism

Each `Error` class now includes a set of suggestions, and these are printed by
the top-level handler.

src/libutil/suggestions.hh

@@ -0,0 +1,41 @@
+#pragma once
+
+#include "comparator.hh"
+#include "types.hh"
+#include <set>
+
+namespace nix {
+
+/**
+ * A potential suggestion for the cli interface.
+ */
+class Suggestion {
+public:
+    int distance; // The smaller the better
+    std::string suggestion;
+
+    std::string pretty_print() const;
+
+    GENERATE_CMP(Suggestion, me->distance, me->suggestion)
+};
+
+class Suggestions {
+public:
+    std::set<Suggestion> suggestions;
+
+    std::string pretty_print() const;
+
+    Suggestions trim(
+        int limit = 5,
+        int maxDistance = 2
+    ) const;
+
+    static Suggestions bestMatches (
+        std::set<std::string> allMatches,
+        std::string query
+    );
+
+    Suggestions& operator+=(const Suggestions & other);
+};
+
+}