replace `cstr!` with const constructor

[japaric]

the const constructor performs, at compile time, the checks that the macro was doing at runtime

[github-actions[bot]]

Number of dependencies and binary size impact report

Metric	main	PR #756	Delta
Direct dependencies	3	3	-
Total dependencies	4	4	-
Binary size	1023.9 KiB	1023.2 KiB	-
Text size	594.9 KiB	594.9 KiB	-

Dependencies diff

```diff └─ sudo-rs [v0.2.0] ├─ glob [v0.3.1] ├─ libc [v0.2.147] └─ log [v0.4.20] ```

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +0.15% :tada:

Comparison is base (0ac6fa8) 54.86% compared to head (348a012) 55.01%. Report is 13 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #756 +/- ## ========================================== + Coverage 54.86% 55.01% +0.15% ========================================== Files 71 71 Lines 9731 9757 +26 ========================================== + Hits 5339 5368 +29 + Misses 4392 4389 -3 ``` | [Files Changed](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety) | Coverage Δ | | |---|---|---| | [src/visudo/mod.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3Zpc3Vkby9tb2QucnM=) | `0.00% <0.00%> (ø)` | | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/756/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety)

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