Adjust Punycode overflow checks

[hsivonen]

• The change made in 1.0.0 incorrectly assumed that the input length limit removed the need to do overflow check when decoding. Now the internal-caller length limit is taken as a permission to skip overflow checks only when encoding.
• The RFC gives overflow checking pre-flight math for languages like that don't have checked math. Since Rust does, the code now uses checked_add and checked_mul instead of pre-flight when overflow checks are performed.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (idna-v1x@dcfbed3). Learn more about missing BASE report.

Files	Patch %	Lines
idna/src/punycode.rs	95.65%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## idna-v1x #949 +/- ## =========================================== Coverage ? 79.89% =========================================== Files ? 23 Lines ? 4227 Branches ? 0 =========================================== Hits ? 3377 Misses ? 850 Partials ? 0 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.