Allow boolean as default parameter

[charles-cooper]

What I did

Fix #1464

How I did it

How to verify it

Description for the changelog

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[jacqueswww]

Add test and change and this can be merged :)

[charles-cooper]

@jacqueswww I agree that using is_literal is more natural, but the values for defaults don't get parsed until much later (parse_[private,public]_function.py), I am hesitant to move the type checker code to that stage, and I also don't want to do extra calls to parse to LLL since there may be unintended side-effects. Maybe I can introduce a function for determining if an AST node is a literal?

[charles-cooper]

It's also a little awkward, because I don't think the current check descends into the AST node anyways. For example this compiles:

x: int128

@public
def test(a: bool = True) -> bool:
    return a

@public
def test2(a: int128[3] = [1, self.x, 3]):
    pass

[jacqueswww]

@charles-cooper you are right, let's leave as is.