Make `siftUp` and `siftDown` public and edit `add` in the `std.PriorityQueue`.

[ANDRVV]

Summary

As for siftUp and siftDown they should be handled by the user too. The solution is to make them public.
In cache-like systems where an item has been taken you need to do a siftUp to update the priority. Flexibility and control over it is important.

It is also needed in the function addSiftUp that it replaces add. This is because there are some contexts where add should not do a siftUp.

It is not a proposal: the std.PriorityQueue function is just that. And having these limits is an implementation error.

💡 Proposed Solution

pub fn add(self: *Self, elem: T) !void
pub fn addSiftUp(self: *Self, elem: T) !void
pub fn siftUp(self: *Self, index: usize) void
pub fn siftDown(self: *Self, index: usize) void