Collectors: Introduce PlainString and make use of it in SQLString
Modify Visitors::Dot's API to use collectors
- Test noop when passing empty list to `insert`
- Rename into test to indicate it accepts table and chains on it
- Rename "combo" test to what it actually tests
Merge pull request #293 from vipulnsward/cleanup-select-manager

Cleanup Insert Manager
Merge pull request #292 from mekishizufu/dot_visitor_api

Modify Visitors::Dot's API to use collectors