Inequality slicing without ..: x[<i] etc.

[edemaine]

As suggested by @STRd6 on Discord

[STRd6]

We should remove the x is and + from these examples because === and + on slices aren't how one may expect.

[JayXon]

This is a little confusing because I thought x[>=i] is a reverse slice where .. is omitted

[edemaine]

We discussed this some in Discord (feel free to join us!).

This PR's notation is more like the binary operator sections (< i) and (>= i), and clearly specify the desired index range.

We could of course also spell x[>=i] as x[i<=] (which wouldn't have the same confusion), and I'd be open to adding that as an alternative spelling. But I worry about making x[>=i] a reverse slice: my feeling is that reverse slices are a niche feature, so don't need concise notation, and I don't think it's particularly intuitive that x[>=i] is reversed, so it's not worth making it so for consistency. But happy to continue discussing!