将处理参数错误的prerr_string调用改为invalid_arg的调用
将处理dotted pair的代码从read_pair剥离出来实现为一个单独的函数
将打印字符对象的代码从write函数中剥离出来实现为函数write_character
预先创建一些符号类型的对象，如quote、set!、define和ok。
创建表示逻辑真和逻辑假的单例对象
将一些重复的car和cdr操作替换为cadxr系列的函数调用
Use the channel data type for inputing.

Replace the use of Stream module in source code.
Replace the `stream' in variable name `in_stream' or `input_stream' to `channel'
Add the vertical bar to first case in pattern matching in block match...with
Remove the use of variable dq

The variable `dq` is used because the ocaml-mode from OCaml 3.12.0
distribution can't recognize the double quote mark correctly.
Remove the definition and apply of function `is_double_quote'
The change of the if...then identation style
Add the first primitive procedure, the plus function.