(0522e2d) Compile an integer to an exectuable that exits with the given number

• 输出一个简单的 echo 汇编到tmp.s.
• Makefile 中的 main.o 默认自动编译，.PHONY声明test和clean为伪目标，防止意外依赖。

(bf7081f) Add + and - operators
(a1ab0ff) Add a tokenizer to allow space characters between tokens