fd/fmt: Add buffered formatting to fd

This makes it possible to trivially write formatted data to a file
descriptor with built-in buffering to reduce the number of syscalls.

Change-Id: Ib66c062b65e2a611f13be570c2ed5fe5eb208fb7
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/meson.build b/test/meson.build
index a0be742..4ffd1a9 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -31,6 +31,7 @@
   gtests += [
     'fd/dupable',
     'fd/managed',
+    'fd/fmt',
     'fd/intf',
     'fd/impl',
     'fd/line',