[5.5] Add a compileForeign method to PostgresGrammar

[challet]

With PostgreSQL driver, the methods Schema::disableForeignKeyConstraints and Schema::enableForeignKeyConstraints are in fact deferring the constraints.

In order for them to be deferred, they need to be deferrable, see https://www.postgresql.org/docs/9.0/static/sql-createtable.html :

Checking of constraints that are deferrable can be postponed until the end of the transaction (using the SET CONSTRAINTS command)

This pull request allows to add such a "DEFERRABLE" option in a migration :

Schema::table('children', function(Blueprint $table) {
  $table
    ->foreign('parent_id')
    ->references('id')
    ->on('parent')
    ->onDelete('cascade')
    ->deferrable('DEFERRABLE INITIALLY IMMEDIATE');
});

Possible values are :

• NOT DEFERRABLE (Postgres default)
• DEFERRABLE
• DEFERRABLE INITIALLY IMMEDIATE (default of the previous line)
• DEFERRABLE INITIALLY DEFERRED

[taylorotwell]

Is there some standard default value we could give this method rather than forcing you to pass a magic string of commands?

[challet]

Yes, but it would need to call two mehods :

• ->deferrable(false) NOT DEFERRABLE (which is the Postgres default if nothing specified)
• ->deferrable(true) or ->deferrable() DEFERRABLE (by Fluent default)
• ->deferrable(true|empty)->initiallyImmediate(true|empty) DEFERRABLE INITIALLY IMMEDIATE (for Postgres it the default of "DEFERRABLE" alone)
• ->deferrable(true|empty)->initiallyImmediate(false) DEFERRABLE INITIALLY DEFERRED

Tests have been added to reflect the different cases.