Remove some panics from zerovec's derive and icu_codepointtrie

[Manishearth]

Discovered whilst working on bionic, I noticed panic machinery being pulled in.

[Manishearth]

The zerovec one seems like a slam dunk to me. The codepointtrie one is iffy: it would be nice to have these panics as debug checks at least. Most Rust release builds do not panic on overflow, however Android does. We could in theory disable that behavior for the bionic binary, but I don't know if it's easy. It is nice to make an attempt to remove panics from this code. On the other hand, it makes the code gnarlier, and bugs will be harder to trace.

(I could introduce some code that wraps the wrapping arithmetic in debug assertions)

Routes to take here:

• Land the second commit, take the complexity cost in code that is seldom looked at. Potentially add debug assertions
• Land the second commit as a patch in bionic.
• Don't worry too much about codesize in bionic, the owners are already fine with the 230KiB delta with panicking stuff (as opposed to 90 KiB without)
• See if we can get bionic to build without overflow panics

We could also potentially add CI ensuring that panicking infra never makes it into bionic.

[sffc]

Good work. Just some small things

[sffc]

Suggestion: Given that we have a good GIGO code path, I agree with removing the panicky subtract impls in "safe release" mode like Android is using. However, I think we should keep the debug asserts in test mode. In other words, since we do this a lot in this file, please make a helper function or macro that does the wrapping_add or wrapping_sub with an explicit debug_assert and use it everywhere in this file.

The helper function/macro could eventually make its way into the shared-macro helper file. (#4467)

[sffc]

I prefer this option conditional on the "potentially" part:

Land the second commit, take the complexity cost in code that is seldom looked at. Potentially add debug assertions

[Manishearth]

Works for me, happy to add debug assertions

[Manishearth]

I managed to have a nice-looking but simple macro; and I also discovered another place in zerovec derive that could use a non-panicking bounds check.