container/heap: add simple example

[rsc]

The priority queue example is fine but very complex. I have heard of people copying and
pasting it (because it is unclear how to use the package otherwise) and having all the
code to maintain those index fields even if they were never used.

Two things to do:

1) Write a simple example. Make it first.
2) Make the complex example actually use the functionality the index field is allowing.

[kisielk]

Comment 1:

For the simple example, how about a simple int heap like so:
http://play.golang.org/p/YxDSwLWqhs ?

[rsc]

Comment 2:

Labels changed: added size-m.

[rsc]

Comment 3:

Labels changed: added suggested.

[gopherbot]

Comment 4 by vegacom:

The Push could be simplified by using append, as in the following code snippet:
func (pq *PriorityQueue) Push(x interface{}) {
  item := x.(*Item)
  item.index = len(*pq)
  *pq = append(*pq, item)
}

[cespare]

Comment 5:

First pass attempt:
https://golang.org/cl/7068048/
I based the simple example on the int heap from Comment 1.

[rsc]

Comment 6:

This issue was closed by revision 0e74f04.

Status changed to Fixed.