libpqxx 7.2.1: bug fixes, deprecations, and tweaks

Here's a new patch release. Changes are:

• Fix infinite loop in converting char * to string. (#377)
• Deprecated namedclass.
• Convert an entire row using row::as<type...>().
• Internal rework of field::to() and field::as() functions.
• Some more warning options in maintainer mode.
• Removed the old, DocBook-based tutorial.
• Fixed wrong query and SQLSTATE params to some exceptions. (#378)

Okay, okay, there's one nice new feature there. You can now convert an entire result row to client-side types at once. See row::as().