[stdlib] Use Intable in getitem and setitem consistently

[JoeLoser]

Many __getitem__ and __setitem__ functions still take an Int as an argument. This is OK, but forces call sites to explicitly construct an Int via int(my_value) for example to call __getitem__ or __setitem__. Instead, we can just make __getitem__ and __setitem__ take an Intable thing that always does the int(value) internally rather than pushing it onto all the callers.

One thing to note is for floating point indices, like x[0.1], this would erroneously call x[int(0.1)] i.e. x[0]. This is surprising and instead should be a hard error. But we can put that on hold and stage this in phases:

1. Change all the __getitem__ and __setitem__ functions to work on Intable and ignore the floating point indexing issue mentioned above.
2. Introduce an Integral trait or something to ensure that callers can't pass a floating point value to these functions. Use that instead of Intable everywhere.
3. Audit other functions using Intable in case they also have floating point indexing issues like mentioned above.

FYI @itramble

[gabrieldemarmiesse]

The python community already solved this issue of floating points :) See the __index__() docs

This is what is called internally when doing my_list[some_object], this is also why my_list[4.2] won't work in python.

Maybe it would be worth it to create a operator module? Then we can call operator.index(x) instead of x.__index__() since dunder method are not supposed to be public

Some real world examples:

• https://github.com/google/trax/blob/a6a508e898a69fecbcce8e5b991666632c629cb0/trax/tf_numpy/numpy_impl/arrays.py#L197
• https://github.com/google/trax/blob/a6a508e898a69fecbcce8e5b991666632c629cb0/trax/tf_numpy/numpy_impl/arrays.py#L197

[bgreni]

I'd like to work on this if it's still available!

[gabrieldemarmiesse]

@bgreni I don't think you need to ask permission to work on an issue. It's quite uncommon for two contributors to work on the same thing. If you don't see a PR open, it's likely that no one is working on it :)