Fallback sorting on labels

[mgorny]

Effort to add the fallback sort on variant label. After much testing, I've figured out that the only sane way to do this without having to change all the API is to add label to VariantDescription. For now, we're not verifying it yet, since there's still tons of call sites to update.

[mgorny]

Okay, this is bigger than I initially planned but I think it's better to make it more future-proof.

The key changes are:

1. VariantDescription now carries the label in addition to variant properties. If it's not provided explicitly, we default to self.hexdigest or null.
2. The separate label argument to the API functions is now deprecated, but still supported — in which case it overrides the label found in VariantDescription. However, for future code we expect users to set label on VariantDescription.
3. Consequently, the get_variant_label() helper is now deprecated in favor of VariantDescription.label.
4. The sort algorithm is updated to take variant label as the final sort key, per the PEP. I've added extra tests for that.
5. I've updated all the tests to explicitly pass label= when constructing non-null VariantDescription. This is not technically necessary now; I've temporarily made that a requirement to make sure I didn't miss any call site. That said, I suppose the tests are more future-proof that way, in case we ever make labels required.
6. In data types that carry the label explicitly (dicts), we preserve the original format but aim to sync the label between the keys and VariantDescriptions.