Replace redundant trait bounds with Model

[ISibboI]

This is as discussed in #89.

I replaced the bounds of the form Model + X with just Model. In the Model trait I wrote a sentence of documentation explaining why the additional trait bounds are needed.

In the ModelCursor struct, instead of writing out the bounds required for Cursor, I restricted the whole struct to Model. That made sense to me, because a ModelCursor seems to be designed to be used with a Model.

I also removed the PhantomData from ModelCursor, since the generic type T is now used in the Cursor.

I additionally added a few characters of documentation stating that we now also test against mongodb 5, and I ran cargo update.

[ISibboI]

Could you merge this and publish it to crates.io? Even doing something like a 0.10.0-alpha.0 would be great already :)

[chumaumenze]

Will this be merged? When will this and https://github.com/thedodd/wither/pull/89 be released?