derived Deserializer methods are slow to compile

[jyn514]

Here is cargo llvm-lines --lib run on docs.rs: https://gist.github.com/jyn514/3a99fca8e1d3a8048d6bba409b76b381

Note 3 of the top 10 functions are all from serde_json, despite there only being ~20 copies:

  Lines          Copies        Function name
  -----          ------        -------------
  621273 (100%)  15169 (100%)  (TOTAL)
   10599 (1.7%)     17 (0.1%)  <&mut serde_json::de::Deserializer<R> as serde::de::Deserializer>::deserialize_struct
    7930 (1.3%)     26 (0.2%)  <serde_json::de::SeqAccess<R> as serde::de::SeqAccess>::next_element_seed
    4845 (0.8%)     17 (0.1%)  <serde_json::de::MapAccess<R> as serde::de::MapAccess>::next_key_seed

This makes up a full 4% of the lines in the whole 15k line project. Is it at all possible to reduce the size of this functions?

In case it helps, here are some example derives:

https://github.com/rust-lang/docs.rs/blob/master/src/utils/cargo_metadata.rs#L65-L66
https://github.com/rust-lang/docs.rs/blob/master/src/utils/cargo_metadata.rs#L109-110
https://github.com/rust-lang/docs.rs/blob/master/src/utils/cargo_metadata.rs#L130-L131