Sql: extend supported syntax

Here are some related POC proposals for extending the syntax supported by the Sql class to improve it as a standalone query builder:

- [X] Replace named params with positional params to allow repeat references, see issue #30
- [X] Add ability to use open/close parentheses in any part of a statement to allow grouping and nesting
- [ ] Add support for nested subqueries

I'll leave these as separate commits for your convenience.