Make append buffer contiguous and update iterators

include/vase/buffer/append.h

@@ -5,19 +5,19 @@
 #include "pch.h"
 
 struct AppendBuffer : Buffer {
-  using TChunk = char[APPEND_CHUNK_SIZE];
-  std::vector<TChunk *> buf;
-  TChunk *t_current;
-  uint64_t current_offset{0};
-  uint64_t t_offset{0};
-
-  AppendBuffer();
+  char *buf = nullptr;
+  uint64_t allocated_capacity = 0;
+  uint64_t current_size = 0;
+  int fd = -1;
 
+  AppendBuffer(std::filesystem::path base_dir);
   ~AppendBuffer();
 
-  uint64_t append(char c);
-  uint64_t append(const char *text, uint64_t length);
+  uint64_t append(const char c);
+  uint64_t append(const char *text, uint64_t len);
+  const char *read(uint64_t pos) override;
+  uint64_t length() override;
 
-  const char *read(uint64_t pos, uint64_t *out_len);
-  inline uint64_t length();
+private:
+  void grow(uint64_t len);
 };