Buffer type compute timer

[MichaelXavier]

I notice that dhall-buffer-type-compute-timeris run after every change to set a variable that is used in the header line. This seems like it isn't needed when dhall-use-header-line is nil because it won't be displayed anywhere. In fact, even on moderately large files, type computing can be really expensive and make the buffer unusable. It seems to me this timer should not even be run if the type won't be used.

Also a lower priority but it seems like there could be some control over the debouncing feature you have which is currently hardcoded to 1 second right now for cases when you do want the type but you don't want to run it as frequently so it doesn't lock up the editor.

Let me know if you'd be amenable to these changes. I'm not an elisp expert but I could give it a shot.