Slightly more powerful split syntax.

There's two additions to the split syntax that would make my life easier:

1. Allow python integer format like 50_000. This helps with readability and catch off-by-an-order-of-magnitude mistakes more easily. Currently train[:50_000] gives an error.
2. If this is not insanely difficult: Allow for a sequence of slicing, like train[:50_000][:10] that reduces to train[:10]. This would allow simplifying logic in my configuration files a lot, where I often define a minival split like train[:50_000], but have an additional "quickrun" flag that appends [:16] to all split names. Now it requires extra logic, but if the above syntax was allowed, it would be much simpler.

No file or test is named. Start by locating the split-syntax parser and its existing tests, then verify the requested integer-literal and chained-slice forms; done means both forms are accepted with the described resulting split behavior and covered by tests.