Htsql

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* General code cleanup and refactoring.
* Improved emitted SQL for projections and aggregates.
* Changed operator precedence to avoid parentheses around
filtering and projection operators.
* Added a test data generator for HTSQL regression tests
(thank to Yuri Yerokhin).
* Added support for hierarchical headers for `/:txt` and `/:html`
formatters.
* Updated documentation.

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* Ported to MySQL, Oracle and MS SQL Server.
* Added support for projections (`base^kernel`).

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* Merged `/:json` and `/:jsonex` formatters; use `/:jsonex` layout
to avoid CSRF issues.
* Documentation updates.

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* Initial beta release.
* PostgreSQL and SQLite backends are (mostly) supported, the former
more than the latter.
* The SQL translator needs more work, in particular, the term tree,
the frame structure and the serializer.
* Lots of functions are either not implemented or missing a proper
serializer, especially in the SQLite backend.
* Other areas which need more work before the final release: rendering,
introspection, regression tests and documentation.