Support actions on FOREIGN_KEY constraints

[emdash]

I tried to do this in the simplest way possible, though there are things I don't like about it.

• renamed FOREIGN_KEY variant to ForeignKey
• helper function FOREIGN_KEY retains previous behavior
• new helper function FOREIGN_KEY' allows passing ON_UPDATE and ON_DELETE

All actions are supported

• NO_ACTION
• SET_NULL
• SET_DEFAULT
• RESTRICT
• CASCADE

Added a test using CASCADE to verify that the action was added to the foreign key constraint.

[stefan-hoeck]

This looks very good! Thanks a lot for adding this!