Support record updates

[leostera]

The following OCaml currently does not compile:

type t = { b: int; c: bool }

let f () =
  let a = { b=1; c=true } in
  { a with b=2 }

Whereas it should compile to:

f() ->
  A = #{ b => 1, c => true },
  A#{ b := 2 }.

To get this done, we need to look into the extended_expression of the Texp_record value and if it is not None, then use that with the already existing Expr.map_update function from the Ast helpers.