Performance of unmounting a context provider

[ikolobar]

• Check if updating to the latest Preact version resolves the issue - No

Describe the bug
Unmounting a context provider is slow if there are many components subscribed to its context.

To Reproduce

Steps to reproduce the behavior:

1. Go to the stackblitz repro
2. Click "Toggle mounted"
3. Notice that unmounting takes a while.
4. Comment out useContext(MyContext) in Component and repeat. It will be much faster.

Expected behavior
It should be faster.

Root cause
Subscribing to a context provider overrides componentWillUnmount:

	        this.sub = function (c) {
	          subs.push(c);
	          var old = c.componentWillUnmount;

	          c.componentWillUnmount = function () {
	            subs.splice(subs.indexOf(c), 1);
	            if (old) { old.call(c); }
	          };
	        };

When unmounting the entire tree, repeatedly splicing the subs array is slow because it shifts the array. If the provider itself is unmounting, it could instead immediately remove all its subs. Alternatively if changing the render order of subscribed components isn't a breaking change, it could avoid the splice cost like this:

            this.sub = function (c) {
              subs.push(c);
              var old = c.componentWillUnmount;

              c.componentWillUnmount = function () {
                var index = subs.indexOf(c);
                var last = subs.pop();
                if (last !== c){
                    subs[index] = last;
                }
                if (old) { old.call(c); }
              };
            };