libpqxx 8.0.2: Small fixes

As you can expect with such a big x.0 release, there are a few things that still needed work. It's been difficult health-wise but I just had to get 8.0 out the door, since most of the work was already done before I fell ill.

The libpqxx community has really stepped up, finding the problems and contributing fixes. I am immensely grateful for this, and more than a little proud.

Fixes to the build:

• The params constructor didn't necessarily recognise a conversion_context argument correctly. (#1237)
• Detection of supported compiler flags was broken.
• include/pqxx/config-compiler.h is now called include/pqxx/internal/config.h. (#1211)

Fixes to the library's behaviour:

• Redesigned which errors inside a transactor perform() will retry. (#1248)
• Empty binary data, passed as a parameter, could accidentally be passed as a null instead. (#1242).
• Some array conversions were under-budgeted, leading to failures. (#1235)
• Conversion from bool to string didn't reserve enough buffer space. (#1229)
• Looks like a memcpy() copied from/to overlapping memory ranges.

Also, there is now a simple benchmark tool for comparing the performance of different ways to retrieve data from the database. It's still very limited.