Refactor the "Pyramid of Doom" out of all the AST node construct classes.

[Atlinx]

Inside of the construct classes instead of chaining expect functions like

var label = parser.expect_token("keyword", "label")
if parser.is_success(label):
    var identifier = parser.expect("identifier")
    if parser.is_success(identifier):
        var expression = parser.expect("expression")
        if parser.is_success(expression):
            ...
        else:
            return expression
    else:
        return identifier
else:
    return label

they can be replaced with

var label = parser.expect_token("keyword", "label")
if not parser.is_success(label):
    return label

var identifier = parser.expect("identifier")
if not parser.is_success(identifier):
    return identifier

var expression = parser.expect("expression")
if not parser.is_success(expression):
    return expression

...

Notice how just by switching the is_success() to a not is_success() check we can break apart the "Pyramid of Doom"