fd/fmt: Support direct string writing

Change-Id: Id49ba3aef72fdc583dbd1b1defe5a0d8b107b238
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/fd/fmt.cpp b/test/fd/fmt.cpp
index 397fcae..bdac7a4 100644
--- a/test/fd/fmt.cpp
+++ b/test/fd/fmt.cpp
@@ -22,18 +22,17 @@
     auto fd = ManagedFd(CHECK_ERRNO(memfd_create("test", 0), "memfd_create"));
     {
         FormatBuffer buf(fd, 4096);
-        buf.append("hi\n");
-        buf.append("hi\n"sv);
+        buf.appends("hi\n", "hi\n"sv);
         EXPECT_EQ(0, fd.lseek(0, Whence::Cur));
         buf.flush();
 
         EXPECT_EQ(6, fd.lseek(0, Whence::Cur));
         buf.append(FMT_COMPILE("{}"), std::string(2050, 'a'));
         EXPECT_EQ(6, fd.lseek(0, Whence::Cur));
-        buf.append(FMT_COMPILE("{}"), std::string(2050, 'a'));
+        buf.append("{}", std::string(2050, 'a'));
         EXPECT_EQ(4106, fd.lseek(0, Whence::Cur));
 
-        buf.append("hi\n");
+        buf.appends("hi\n");
         EXPECT_EQ(4106, fd.lseek(0, Whence::Cur));
     }
     EXPECT_EQ(4109, fd.lseek(0, Whence::Cur));