str/buf: Support std::back_inserter and data()

Change-Id: I07cc8067da62fbe4607d4dc2cf54dc25f559679a
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/str/conv.cpp b/test/str/conv.cpp
index 3a2f41c..969258d 100644
--- a/test/str/conv.cpp
+++ b/test/str/conv.cpp
@@ -103,7 +103,7 @@
     StrBuf buf;
     ToStrAdap<ToStr<TestValS>>{}(buf, TestValS{});
     EXPECT_EQ("test", buf);
-    buf.reset();
+    buf.clear();
     auto ptr = buf.append(4);
     EXPECT_EQ(4, ToStrAdap<ToStr<TestValS>>{}(ptr, TestValS{}) - ptr);
     EXPECT_EQ("test", std::string_view(ptr, 4));