Add query reborrowing

[SkiFire13]

Objective

• Sometimes some method or function takes an owned Query, but we don't want to give up ours;
  • transmuting it technically a solution, but it more costly than necessary.
• Make query iterators more flexible
  • this would allow the equivalent of slice::split_first/slice::split_first_mut for query iterators
  • helps with requests like Is there a way to iterate through Query results in the same way as a Vec, or to get a Vec of entities? #14685

Solution

• Add a way for reborrowing queries, that is going from a &'a mut Query<'w, 's, D, F> to a Query<'a, 's, D, F>:
  • this is safe because the original query will be borrowed while the new query exists and thus no aliased access can happen;
  • it's basically the equivalent of going from &'short mut &'long mut T to &'short mut T the the compiler automatically implements.
• Add a way for getting the remainder of a query iterator:
  • this is interesting also because the original iterator keeps its position, which was not possible before;
  • this in turn requires a way to reborrow query fetches, which I had to add to WorldQuery.

Showcase

• You can now reborrow a Query, getting an equivalent Query with a shorter lifetime. Previously this was possible for read-only queries by using Query::to_readonly, now it's possible for mutable queries too;
• You can now separately iterate over the remainder of QueryIter.

Migration Guide

• WorldQuery now has an additional shrink_fetch method you have to implement if you were implementing WorldQuery manually.

[re0312]

I really like this . and from a performance perspective, except for some specific fetch , it's almost zero cost.
I did a simple bench and it was nearly 2X faster than our QueryCombinationIter<2>