Implement strbuffer

String buffer (strbuffer) is an object that resizes automatically when
data is added to it. It was implemented by generalizing the technique
used in json_dumps().

src/strbuffer.h

@@ -0,0 +1,19 @@
+#ifndef STRBUFFER_H
+#define STRBUFFER_H
+
+typedef struct {
+    char *value;
+    int length;
+    int size;
+} strbuffer_t;
+
+void strbuffer_init(strbuffer_t *strbuff);
+void strbuffer_close(strbuffer_t *strbuff);
+
+char *strbuffer_value(strbuffer_t *strbuff);
+char *strbuffer_steal_value(strbuffer_t *strbuff);
+
+int strbuffer_append(strbuffer_t *strbuff, const char *string);
+int strbuffer_append_bytes(strbuffer_t *strbuff, const char *data, int size);
+
+#endif