refactor: use cursor instead of BufferReader for input format

[youngsofun]

1. currently, also usage is MemoryReader (with all data already readn).
2. in the future, async is slow, the current BufferReader interfaces are all sync; while we are not likely to read the underlying Reader syncly
3. not assuming any underlying Reader save a lot of cost
   4. most of all, no need to alloc and copy for the buf
   1. not need loop and the call to fill_buffer.
   2. not need to return Result and than unwarp for most fn.
   3. checkpointing is much easier, no need to maintain a copy of data.

also prepare for new expression