Move hiliteMatches into a separate header

This is mostly so that we don't #include <regex> everywhere (which
adds quite a bit of compilation time).

src/libutil/hilite.cc

@@ -0,0 +1,44 @@
+#include "hilite.hh"
+
+namespace nix {
+
+std::string hiliteMatches(
+    std::string_view s,
+    std::vector<std::smatch> matches,
+    std::string_view prefix,
+    std::string_view postfix)
+{
+    // Avoid copy on zero matches
+    if (matches.size() == 0)
+        return (std::string) 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;
+}
+
+}