pw_kvs: Rename output.h to io.h

Prepare for introducing an Input interface.

Change-Id: I6582adbf139577cc1e81110ac952508fa767fa9e
diff --git a/pw_kvs/BUILD b/pw_kvs/BUILD
index 218d199..e08c214 100644
--- a/pw_kvs/BUILD
+++ b/pw_kvs/BUILD
@@ -44,8 +44,8 @@
         "public/pw_kvs/crc16_checksum.h",
         "public/pw_kvs/flash_memory.h",
         "public/pw_kvs/format.h",
+        "public/pw_kvs/io.h",
         "public/pw_kvs/key_value_store.h",
-        "public/pw_kvs/output.h",
     ],
     includes = ["public"],
     deps = [
diff --git a/pw_kvs/BUILD.gn b/pw_kvs/BUILD.gn
index d8396b5..519ee3b 100644
--- a/pw_kvs/BUILD.gn
+++ b/pw_kvs/BUILD.gn
@@ -26,8 +26,8 @@
     "public/pw_kvs/checksum.h",
     "public/pw_kvs/flash_memory.h",
     "public/pw_kvs/format.h",
+    "public/pw_kvs/io.h",
     "public/pw_kvs/key_value_store.h",
-    "public/pw_kvs/output.h",
   ]
   sources = [
     "alignment.cc",
diff --git a/pw_kvs/public/pw_kvs/alignment.h b/pw_kvs/public/pw_kvs/alignment.h
index 0d734d5..98f3d26 100644
--- a/pw_kvs/public/pw_kvs/alignment.h
+++ b/pw_kvs/public/pw_kvs/alignment.h
@@ -18,7 +18,7 @@
 #include <initializer_list>
 #include <utility>
 
-#include "pw_kvs/output.h"
+#include "pw_kvs/io.h"
 #include "pw_span/span.h"
 #include "pw_status/status_with_size.h"
 
diff --git a/pw_kvs/public/pw_kvs/output.h b/pw_kvs/public/pw_kvs/io.h
similarity index 100%
rename from pw_kvs/public/pw_kvs/output.h
rename to pw_kvs/public/pw_kvs/io.h