- Introduce vtab subpackage (modernc.org/sqlite/vtab) exposing Module, Table,...

See merge request !84

The previous code was subject to a TOCTOU race:
the atomic.AddInt32 in the select statement could succeed,
but then context switch away before actually calling c.interrupt.
The actual call to c.interrupt could end up occurring later,
interrupting an unrelated subsequent query.

Fixes #241

fix TOCTOU interrupt race

Closes #241

See merge request !86