Add recursive data cost to fuzzer + speed up `deserialize_any` identifier check

[juntyr]

Adding layers, e.g. enum variants or tuples, is not without cost in the format, so let's enforce that in the fuzzer as well.

Checking for an identifier in deserialize_any previously used the identifier parser, which has complex error handling to produce nice error messages. Since we only need to check if one is there, we now use skip_identifier, which now returns Option<&str> instead of bool, and thus avoids any allocation in the check.

* [ ] I've included my change in CHANGELOG.md

• [x] Fix the code coverage regression

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (817407c) 100.00% compared to head (10f1e97) 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #506 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 78 78 Lines 11053 11063 +10 ========================================= + Hits 11053 11063 +10 ``` | [Files Changed](https://app.codecov.io/gh/ron-rs/ron/pull/506?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/de/mod.rs](https://app.codecov.io/gh/ron-rs/ron/pull/506?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2RlL21vZC5ycw==) | `100.00% <100.00%> (ø)` | | | [src/parse.rs](https://app.codecov.io/gh/ron-rs/ron/pull/506?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL3BhcnNlLnJz) | `100.00% <100.00%> (ø)` | | | [tests/438\_rusty\_byte\_strings.rs](https://app.codecov.io/gh/ron-rs/ron/pull/506?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-dGVzdHMvNDM4X3J1c3R5X2J5dGVfc3RyaW5ncy5ycw==) | `100.00% <100.00%> (ø)` | |

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