add record-style update syntax

[simongregersen]

This MR adds update syntax similar to the syntax offered for record creation when storing functions. Instead of writing

x<| field ::= fun n => plus m n |> 

you can now write

x<| field n := plus m n |> 

It can be quite convenient when the update you're trying to apply cannot be easily curried (e.g. due to other notation).

Custom Coq notation is still a bit black magic to me (in particular which levels to use...) but all tests pass and it seems to work as intended. I've added the new syntax as only parsing to cause the least amount of disruption.