Add execute() to the SelectInterface

Makes total sense to have it in the interface. I also do not know of exceptions so please enlighten us or get it in :)

Let's avoid the following :

For the other methods, look at: #2168241: Type hints for optional methods in StatementInterface (D8) / DatabaseStatementInterface (D7)

I believe it was left off because execute() is a method on the Query base class, which all query builders extend. In hindsight, that seems like a rather stupid reason. :-)

This is *probably* D7-backport-safe. (It only would break on a select object that doesn't implement execute(), which would already be pointless).

@Crell:
The reason why this will make problems in D7 for PHP 5.3.8 and older:
http://3v4l.org/5T5vv
http://stackoverflow.com/questions/17525620/php-fatal-error-cant-inherit...
https://bugs.php.net/bug.php?id=43200

Sigh PHP...

Btw, DatabaseStatement::execute() is already covered in the other issue #2168241: Type hints for optional methods in StatementInterface (D8) / DatabaseStatementInterface (D7) (although not exactly like here), but this other issue is doing too many things at once. So probably a good idea to get this in first, and then slice up the other issue into smaller ones.

Committed/pushed to 8.x, thanks!

What about http://3v4l.org/viggJ

interface QueryInterface {
    function execute();
}

interface SelectInterface extends QueryInterface {}

abstract class Query implements QueryInterface {}

class Select extends Query implements SelectInterface {
    function execute() {}
}