AST YAML printing

[inmaldrerah]

This PR supports a new convenient feature: printing an AST into YAML format. This may be helpful for debugging.

A YamlPrinter is defined to print any AST Node to YAML format. Node also has a yamlPrint() convenient method to print its contents to YAML format directly.

For example, the following code:

func factorial(n: Int64): Int64 {
    if (n <= 1) {
        1
    } else {
        n * factorial(n - 1)
    }
}

main() {
    factorial(5)
}

is printed as:

type: Program
decls:
- type: FuncDecl
  keyword: func # Position: 1:5
  identifier: factorial # Position: 1:15
  declType:
    type: PrimitiveType
    typeParameterName: factorial # Position: 1:15
    typeName: Int64 # Position: 1:32
  params:
  - type: FuncParam
    declType: null
    name: n # Position: 1:17
    paramType:
      type: PrimitiveType
      typeParameterName: n # Position: 1:17
      typeName: Int64 # Position: 1:24
  block:
    type: Block
    nodes:
    - type: IfExpr
      condition:
        type: BinaryExpr
        left:
          type: RefExpr
          identifier: n # Position: 2:10
        oper: '<=' # Position: 2:13
        right:
          type: LitConstExpr
          kind: 'integer literal'
          literal: 1 # Position: 2:15
      ifBlock:
        type: Block
        nodes:
        - type: LitConstExpr
          kind: 'integer literal'
          literal: 1 # Position: 3:10
      elseBranch:
        type: Block
        nodes:
        - type: BinaryExpr
          left:
            type: RefExpr
            identifier: n # Position: 5:10
          oper: '*' # Position: 5:12
          right:
            type: CallExpr
            callee:
              type: RefExpr
              identifier: factorial # Position: 5:22
            arguments:
            - type: Argument
              expr:
                type: BinaryExpr
                left:
                  type: RefExpr
                  identifier: n # Position: 5:24
                oper: '-' # Position: 5:26
                right:
                  type: LitConstExpr
                  kind: 'integer literal'
                  literal: 1 # Position: 5:28
- type: MainDecl
  declType: null
  param: null
  block:
    type: Block
    nodes:
    - type: CallExpr
      callee:
        type: RefExpr
        identifier: factorial # Position: 10:14
      arguments:
      - type: Argument
        expr:
          type: LitConstExpr
          kind: 'integer literal'
          literal: 5 # Position: 10:16