bugfix for If-else, Head (nil), Let-in
If-else : rewrite type checking
Head: dealing with nil in list
Let-in: ignore the typeMap in env stack
pass 8-queen test, cheers!
syntax parser:  assignment + ...
Merge branch 'master' of https://github.com/zetan/SimPLPope
modify the error in lexer add add the Eval function in NOP