Support for arbitrary size integers

[kejace]

I choose to embed the Integers into the Numeric postgres type.
I'm not sure this is the right thing to do so I'm opening this PR for discussion.

[valderman]

This would indeed be nice for PostgreSQL, but what about SQLite? As a general rule, if something works out of the box without importing Database.Selda.PostgreSQL, then it should work with all backends.

A possible solution would be to move the SqlType instance declaration for Integer into selda-postgresql to ensure that one doesn't accidentally use it with the SQLite backend, and mark the LInteger and TInteger constructors as being clearly intended for PostgreSQL use only (by renaming to L/TPGInteger or something). Maybe even create a Database.Selda.PostgreSQL.Types module or similar for the instance, to ensure that you need to be really explicit about wanting to use these types.

A proper, long-term solution would be to add a more general notion of backend-specific types, possibly parameterizing SqlType over a backend, but I think a more hacky implementation of this is fine in the meantime.

[TomMD]

Seeing as the discussion on Integer support, both here and in the issue tracker, was negative perhaps this should be closed?