Renaming fields

It would be nice to be able to write something like this:
```ocaml
type person = {
first : string;
last : string;
age : int;
}
type individual = {
given : string;
family : string;
age : int;
} [@@deriving stable_record ~version:person ~rename:[(given, first); (family, last)])
```

Use the OCaml example with [@@deriving stable_record ...] as the starting case. Locate the stable_record deriving implementation and existing version-handling behavior, then verify that the given/family to first/last mappings compile and conversions work as shown.

Written by the indexing model from the issue text.