Start C++ rewrite.

abuf.cc

@@ -0,0 +1,17 @@
+#include "abuf.h"
+
+namespace ke {
+
+void abuf::append(std::string_view s) {
+    buffer_.append(s);
+}
+
+void abuf::append(const char* s, std::size_t len) {
+    buffer_.append(s, len);
+}
+
+void abuf::append(char c) {
+    buffer_.push_back(c);
+}
+
+} // namespace ke