Add support for arithmetic expression and if else statement

[1001mei]

• Add compiler support for followings (unless otherwise stated, all applicable only to int type):
  • local variable declaration
  • arithmetic expression involving only binary operators
  • println method that takes String or int type argument
  • (nested) if else statements
  • while and do while loops
  • for loop with the form for (init; cond; update) {}
  • condition expression involved in if/else if/while/for can only have int or boolean literal, and can support all logical operators except logical not (!)
• Tests will be added in another PR to avoid huge PR and ease process of reviewing

[Chang-CH]

LGTM

[xyliew25]

Gave some thoughts regarding the AST types with reference to previous discussions.

[xyliew25]

Thanks for the changes, LGTM!