Remove libpqtypes, use plain libpq

[arybczak]

Drop the bundled libpqtypes C library. Queries are now executed with the API of plain libpq and the binary transport format is handled by the code based on postgresql-binary package (with some bugs fixed).

The library now supports decoding of arbitrary composite (row) types (in particular anonymous ones), named composite types are no longer necessary (and thus no longer have to be registered up-front).

Support for enum types is also improved, single values don't need casts anymore when passed as parameters (arrays of enums still do unfortunately).

Benchmark results against master:

  insert:                       OK
    407  ms ±  24 ms, 23% less than baseline
  select parents:               OK
    42.2 ms ± 1.5 ms, 51% less than baseline
  select parents with children: OK
    95.1 ms ± 2.2 ms, 55% less than baseline
  select big array:             OK
    9.37 ms ± 822 μs, 20% more than baseline
  select many rows:             OK
    11.8 ms ± 383 μs, 57% less than baseline

Selection of big arrays of primitive types (4th benchmark, 100000 elements of int4) is now slightly slower, but this is an edge case that we don't really make use of. I actually managed to make this faster than baseline, but the additional code complexity wasn't IMO worth it.