Implemented lz_sb_to_cstr

examples/sb_to_cstr.c

@@ -0,0 +1,16 @@
+#include <stdio.h>
+#define LAZY_IMPL
+#include "../lazy.h"
+
+int main() {
+  Lz_SB x = lz_sb_from_cstr("Hello");
+  Lz_SB y = lz_sb_from_cstr(" World");
+
+  lz_da_concat(x, y);
+  lz_sb_add_char(&x, '\n');
+
+  char *s = lz_sb_to_cstr(&x);
+  printf("%s", s);
+  free(s);
+  return 0;
+}