deserialize_str with escaped strings is broken

[jordens]

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serde_json_core needs to refuse deserializing a borrowed string with escape sequences.

serde_json_core::from_slice::<&str>(br#""\n""#) incorrectly gives Ok(("\\n", 4)).

serde_json::from_slice::<&str>(br#""\n""#) correctly says Error("invalid type: string \"\\n\", expected a borrowed string", line: 1, column: 4).