explicit prepared statement support

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By constraining all of its SQL into a small, pregenerated set of static strings, DBXS makes it easier for drivers like sqlite3 and psycopg whose default prepared statement support is to simply maintain a fixed-size cache of statements.

However, this support is presently accidental. Although those drivers support tuning the cache size, and DBXS has the ability to enumerate all of the queries that all loaded accessors support ahead of time as long as they've all been imported, we don't provide any hints to those drivers. Similarly, drivers like MySQL which require an explicit preparation step are not supported.

DBXS should press this performance advantage more, and add explicit support and testing to ensure that prepared statements are being used everywhere.