Add inline attributes to generated code

[appetrosyan]

Simple constructors and a few other functions are normally never complex (they generate few lines of assembly). Marking those as inline can greatly improve performance for if they are used outside the crate.

Adding inline could greatly improve the performance of libraries that derive_more.

[JelteF]

Afaik this is already done for all derives that are not a trait implementation (like the Constructor derive). If you find one without feel free to make a PR.