Array parsing, and bug fixes.

Adds a first-generation parser for SQL arrays. We'll want to improve on this later. See the pqxx/array header.

This release also fixes some bugs which would break builds under specific circumstances — in some cases builds of the library, in other cases builds of client software. So, if you had trouble compiling with 6.0, try 6.1.

Dependencies between library headers have changed. It's possible that some code may need an extra #include here or there, if your code implicitly relied on some libpqxx headers including other libpqxx headers.