add syntax
add left assoc operator rule
fix left assoc operator
Add class definition parse rule
Add FnDef parse rule
Add StrLiteralNode
Fix compound_stat rule
Fix str() format for StringLiteralNode and KeywordNode
Modify sample
Add fn call
Add class deriving
Modify compound statment
Add if statment
Add while loop
Modify readme
it's kill-time
Fix return stat
Add keyword "else"
Rename Parser::nonTerm
Add for statment