replace serialize key,value functions by serialize_entry

[jmfiaschi]

The purpuse is to serialize serde_json::Map<> with other serializer like serde-xml-rs::to_writer(&mut writer, &json_value) that is not possible today. The problem is serde_json use only map.serialize_key(k) and map.serialize_key(v). Some serializer can use only map.serialize_entry(k, v). If we follow these advices :
https://docs.serde.rs/src/serde/ser/mod.rs.html#1778-1784
/// Serialize a map key.
///
/// If possible, Serialize implementations are encouraged to use
/// serialize_entry instead as it may be implemented more efficiently in
/// some formats compared to a pair of calls to serialize_key and
/// serialize_value.
and the implementation of serialize_entry : https://docs.serde.rs/src/serde/ser/mod.rs.html#1798-1826

We should use during the Map serialize use serialize_entry function instead of serialize_key/serialize_key and use them into serialize_entry.

[dtolnay]

Thanks, published in 1.0.46.