Add minimal support for flattening roundtrips through maps

Supersedes #403, #453, and #454 to add minimal support for #[serde(flatten)] in RON. Instead of many workarounds to make flattened structs serialise into nice RON struct syntax and deserialise from them, both of which are really difficult since serde gives us no information to work with and just delegates to map serialisation/deserialisation, this PR just ensures that at least flattened structs can roundtrip through RON.

For this, just one hack is necessary. If we detect that a map is deserialised into what looks like a struct (thanks @clouds56 for contributing the detection in #454), we enforce that map key identifiers are actually strings written exactly as ", identifier, ". This seems to be enough to get the tests to work that I initially designed for https://github.com/ron-rs/ron/pull/403.

@torkleyy I'd be in favour of landing this instead of going down the rabbit hole of https://github.com/ron-rs/ron/pull/403 since that just fixes one of the two parts and getting flattened structs to serialise correctly would require hacks of another order.

This would not really fix https://github.com/ron-rs/ron/issues/115 but at least get some working if imperfect solution.

[x] Add tests for the new error cases, i.e. when flattening goes wrong
[x] I've included my change in CHANGELOG.md

Codecov Report

Patch coverage: 97.50% and project coverage change: -0.24 :warning:

Comparison is base (484fcab) 87.06% compared to head (bcbb027) 86.83%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #455 +/- ## ========================================== - Coverage 87.06% 86.83% -0.24% ========================================== Files 60 61 +1 Lines 7601 7967 +366 ========================================== + Hits 6618 6918 +300 - Misses 983 1049 +66 ``` | [Impacted Files](https://codecov.io/gh/ron-rs/ron/pull/455?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://codecov.io/gh/ron-rs/ron/pull/455?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2RlL21vZC5ycw==) | `73.73% <80.48%> (-3.19%)` | :arrow_down: | | [src/de/id.rs](https://codecov.io/gh/ron-rs/ron/pull/455?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2RlL2lkLnJz) | `87.65% <100.00%> (-12.35%)` | :arrow_down: | | [tests/115\_minimal\_flattening.rs](https://codecov.io/gh/ron-rs/ron/pull/455?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-dGVzdHMvMTE1X21pbmltYWxfZmxhdHRlbmluZy5ycw==) | `100.00% <100.00%> (ø)` | | ... and [3 files with indirect coverage changes](https://codecov.io/gh/ron-rs/ron/pull/455/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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ron-rs / ron

Add minimal support for flattening roundtrips through maps #455

Codecov Report