[Dbix-class] Cursor and constraint compat added

[Matt S Trout]

DBIx::Class::Cursor now provides CDBI Iterator equivalent behaviour (bar
no support for ->slice yet) but keeps the sth internally and fetches only
as required. ->count is implemented via COUNT(*) for normal search/retrieves
and via fetching everything for set_sql compat.

I've also ported the Class::DBI constraint code as ::CDBICompat::Constraints,
which allows us to pass all but two tests (the column object bits) in the
cdbi constraints.t

This leaves us with the following missing things in the compat layer -

- might_have
- slice for Iterators
- non-database has_a
- TEMP columns
- meta_info
- read-only objects
- column objects

Read-only objects I, quite frankly, couldn't care less about. If you use this
feature please raise your hand and preferably ensure it's holding at least
a good start at a patch :)

Column objects I'd like to leave until we've got Dan Kubb's field
implementation merged - I'd much rather provide the API based on that.

The rest I'll try and get to as I have time; anybody who fancies having a
go is more than welcome to, though.

-- 
     Matt S Trout           Website: http://www.shadowcatsystems.co.uk
  Technical Director        E-mail:  mst (at) shadowcatsystems.co.uk
Shadowcat Systems Ltd.