Move hilite_all into libutil and rename it to hiliteMatches

The signature was also changed so the function now accepts a vector
instead of an iterator

src/libutil/fmt.cc

@@ -0,0 +1,38 @@
+#include <regex>
+
+namespace nix {
+
+std::string hiliteMatches(const std::string &s, std::vector<std::smatch> matches, std::string prefix, std::string postfix) {
+    // Avoid copy on zero matches
+    if (matches.size() == 0)
+        return s;
+
+    std::sort(matches.begin(), matches.end(), [](const auto &a, const auto &b) {
+        return a.position() < b.position();
+    });
+
+    std::string out;
+    ssize_t last_end = 0;
+
+    for (auto it = matches.begin(); it != matches.end();) {
+        auto m = *it;
+        size_t start = m.position();
+        out.append(s.substr(last_end, m.position() - last_end));
+        // Merge continous matches
+        ssize_t end = start + m.length();
+        while(++it != matches.end() && (*it).position() <= end) {
+            auto n = *it;
+            ssize_t nend = start + (n.position() - start + n.length());
+            if(nend > end)
+                end = nend;
+        }
+        out.append(prefix);
+        out.append(s.substr(start, end - start));
+        out.append(postfix);
+        last_end = end;
+    }
+    out.append(s.substr(last_end));
+    return out;
+}
+
+}