fetch_first doesn't work when cursoring is enabled

[eprothro]

If cursor_by has been called on a cassie query class, fetch_first will fail, with:

A singleton method has already been defined for `limit`. `with_limit` can't be implemented.
     # ./lib/cassie/statements/statement/limiting.rb:89:in `assert_no_limit_singleton'
     # ./lib/cassie/statements/statement/limiting.rb:70:in `define_limit_singleton'
     # ./lib/cassie/statements/statement/limiting.rb:46:in `with_limit'
     # ./lib/cassie/statements/execution/peeking.rb:12:in `execute'
     # ./lib/cassie/statements/execution/fetching.rb:25:in `fetch'
     # ./lib/cassie/statements/execution/fetching.rb:38:in `block in fetch_first'