first commit. half-work of type check
Finish Minijava2Piglet, pass all tests.
finish piglet 2 spiglet, pass all the 8 testcase from ucla
finish Kanga 2 MIPS, pass all 8 test case.
finish Spiglet 2 Kanga, pass all the 8 testcases compile Minijava to Mips
finish all five steps, can compile Minijava to MIPS correctly. Pass All 8 testcase.
Merge https://github.com/zlfengyi/Minijava_Compiler
final version