Unify handling of file & inline snapshots

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This is the code for https://github.com/mitsuhiko/insta/issues/456#issuecomment-1979089438, as mentioned in https://github.com/mitsuhiko/insta/pull/466.

There's a very small breaking change in YAML inline snapshots — shown here in the tests. In return, it cuts some complicated code from macros.rs, making them more maintainable and less likely to hit some of our recent issues, such as inconsistent support for trailing commas etc