support

To stay backwards-compatible as much as possible, we should keep all
methods from the soci-simple API and just forward them to their new
equivalents.

Types that refered to non-fixed-size types have been replaced in favor
of their fixed-size types (e.g. dt_integer -> dt_int32, x_short ->
x_int16). To be backwards compatible, we need to keep all of the
previous types though.

bool values occupy 1 byte of storage in Postgres, so we might as well
extract it as 1 byte instead of 4.

Despite previously assumed and tested, most of the databases support
storing the various unsigned integer values. The checks for it in the
tests are therefore obsolete.
Only Firebird and SQLite seem to have a problem with storing UINT64_MAX.
The retrieval of it in a sorted result set with multiple other values
shows that the value is stored as a signed integer in the database.

Firebird and SQLite store UINT64_MAX incorrectly, leading to an incorrect
value ordering when retrieving sorted table contents.

The currently used preprocessor defines used to distinguish between
platforms and defining the corresponding type_conversion/exchange_traits
was found in multiple files. The logic of it is now put in a single place
with custom defines to be used instead.