Fix no_std Support for idna

[domenukk]

This fixes no_std builds for the idna crate.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e4dbb43) 81.20% compared to head (565b94f) 81.20%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #843 +/- ## ======================================= Coverage 81.20% 81.20% ======================================= Files 20 20 Lines 3516 3516 ======================================= Hits 2855 2855 Misses 661 661 ``` | [Impacted Files](https://app.codecov.io/gh/servo/rust-url/pull/843?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=servo) | Coverage Δ | | |---|---|---| | [idna/src/uts46.rs](https://app.codecov.io/gh/servo/rust-url/pull/843?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=servo#diff-aWRuYS9zcmMvdXRzNDYucnM=) | `94.19% <100.00%> (ø)` | |

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[domenukk]

The CI error doesn't seem related to this PR:

error: package `log v0.4.19` cannot be built because it requires rustc 1.60.0 or newer, while the currently active rustc version is 1.56.0

[domenukk]

There's a CI check in place in PR #831