Python's dataclass-like behavior for `@fieldwise_init`

Mojo Language Design
1

Ideally we’d be able to do something like this at some point:

@fieldwise_init
struct MyComplex:
    var re: Int
    var im: Int = 0

@fieldwise_init
struct MyComplexList:
    comptime _L = List[MyComplex]
    var elems: Self._L = DefaultFactory(Self._L.__init__)

comptime DefaultFactory[...] = ... # some mlir black magic

And I think this would also allow us to finally have default values for functions where the values can’t be folded at compile time, which has already happened to me a couple of times and is annoying because I had to redesign the API.

cc: @denis