[5.3] Unify join and builder syntax

[acasar]

This is a massive cleanup of the join clause (~200 lines of production code removed). The idea is to extend the query builder and reuse its functionality instead of duplicating it. This has multiple benefits:

• no need to duplicate the logic for where clauses
• simplified join compilation (by reusing where clause compiler)
• unified syntax between join clause and query builder (i.e. "=" operator is now optional, access to additional clauses like whereRaw, whereSub, ...)

The only breaking change is the removal of $where boolean parameter in the on clause and the removal of some public JoinClause properties.

[willrowe]

Would there be any issues with using methods available on Builder that wouldn't necessarily be valid in a join statement? I'm thinking of methods like select, orderBy, or groupBy.

[acasar]

They will be ignored, same as with nested where... Currently nothing is stopping you from writing:

User::where(function($query){
    $query->orderBy('foo');
})->get();

I first wanted to extract the where builder in a separate class and use it for nested wheres & joins, but it would make merging 5.2 -> 5.3 a nightmare.

The only problem is if you introduce non-where bindings - the query would fail in both cases.

[srmklive]

Totally agree with @acasar in this regard. Seems a really good feature to have in 5.3

[taylorotwell]

Why did you need to change this test?

[acasar]

The last param (true) was removed from the on clause since it no longer makes sense.

[taylorotwell]

Looks good. Thanks!