Add `ChunkedSortedStringSet::first()`.

`LinearSortedStringSet::first()` already exists. The first element is stored
contiguously, so a simple accessor returning `absl::string_view` is possible.

PiperOrigin-RevId: 901838348

riegeli/containers/chunked_sorted_string_set.h

diff --git a/riegeli/containers/chunked_sorted_string_set.h b/riegeli/containers/chunked_sorted_string_set.h
index 0d9cd73..1a080a3 100644
--- a/riegeli/containers/chunked_sorted_string_set.h
+++ b/riegeli/containers/chunked_sorted_string_set.h
@@ -248,6 +248,13 @@
     return chunks_.empty() ? 0 : chunks_.back().cumulative_end_index;
   }
 
+  // Returns the first element. The set must not be empty.
+  absl::string_view first() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    RIEGELI_ASSERT(!empty())
+        << "Failed precondition of ChunkedSortedStringSet::first(): empty set";
+    return chunks_.front().set.first();
+  }
+
   // Returns `true` if `element` is present in the set.
   //
   // If `index != nullptr`, sets `*index` to the index of `element` in the set,

riegeli/containers/linear_sorted_string_set.cc

diff --git a/riegeli/containers/linear_sorted_string_set.cc b/riegeli/containers/linear_sorted_string_set.cc
index 6166de4..3af424d 100644
--- a/riegeli/containers/linear_sorted_string_set.cc
+++ b/riegeli/containers/linear_sorted_string_set.cc
@@ -141,8 +141,7 @@
 absl::string_view LinearSortedStringSet::first() const
     ABSL_ATTRIBUTE_LIFETIME_BOUND {
   RIEGELI_ASSERT(!empty())
-      << "Failed precondition of LinearSortedStringSet::first(): "
-         "empty set";
+      << "Failed precondition of LinearSortedStringSet::first(): empty set";
   const absl::string_view encoded_view = encoded_;
   uint64_t tagged_length;
   const size_t tagged_length_length =