Initialize fallible fields

An initializer can return Result[(), E] when setting up its fields may fail. The constructor then returns Result[Class, E]. Here, reserving list capacity can fail to allocate:

class Buffer:
    values: list[i64]

    def __init__(self: &mut Buffer, size: i64) -> Result[(), AllocError]:
        self.values = list[i64].with_capacity(size)?
        Ok(())

def main() -> Result[(), AllocError | IoError]:
    print(Buffer(8)?)?
    Ok(())

Buffer(values=[])

If initialization fails, the fields already initialized are dropped, but the incomplete instance’s __del__ is skipped. Successful initialization must fill every field and return Ok(()).