Add Lwd.update

[OlivierNicole]

I found myself defining this function quite quickly when trying Lwd, so I think it could be a useful addition. I do not claim that map_and_set is necessarily the best name.

[Julow]

This is also one of the first utility function I defined.

[let-def]

That looks good, thank you.
What do you think about a may_update : ('a -> 'a option) -> 'a var -> unit variant?

[Julow]

Instead, what do you think of not invalidating the document when the new value is the same as the old one ? (I also think that would be a useful property on Lwd.t values #38)

[OlivierNicole]

Why not all of the above?

[let-def]

Thanks, I merged your PR and I will add a may_update function.

[OlivierNicole]

Thanks!