Support nullable foreign keys.

[donut]

I have a table which has a field that is a nullable foreign key. When I try to compile I get this:

I have a table with a primary key that is of type VARCHAR and is referenced by another table, but it looks like Sequoia only supports. At least, when I try to compile I get this:

Error: Unbound value Field.Null.foreign_key

Example:

open Sequoia_mysql

module URL = struct
  include (val table "url")
  let id = Field.int "id"
  (* ... *)
end

module Use = struct
  include (val table "use")
  let id = Field.int "id"
  let url = Field.foreign_key "url" ~references:URL.id
  let referer = Field.Null.foreign_key "referer" ~references:URL.id (* <- error *)
  (* ... *)
end

[andrenth]

There's no support for nullable foreign keys. I could add this, but I'm probably not adding new features until I can figure out a non-hackish way to implement the PPX extension. Currently there are issues when tables and queries are defined in separate files which make the library a bit annoying to use in practice.