Make VEC_BOX lint configurable over the size of T

[flip1995]

In #3545 the VEC_BOX lint was implemented. This lints on usage of Vec<Box<T: Sized>> . The Box around T could make sense though, if T is a very large type. (See also #3530 (comment))

A good enhancement for this lint would be, to make it configurable over the size of T, with some meaningful default value (TBD).

[udoprog]

Just wanted to echo this. I'm storing a lot of large objects in a Vec (~32G in total) and in order to meet memory constraints they need to be boxed. When the vector is at capacity requires the next capacity's worth of contiguous memory to move the values into. Also shuffling the pointers around has proven to be much more efficient.

So the vec_box lint is very noisy for me. Preferably I'd like to be able to mark the contained types for which it should be ignored.