Added support for PostgreSQL 18

Added support for PostgreSQL 15 and 16

• Fixeduint8 on Red Hat-based systems
• Dropped support for 32-bit builds

• Added/fixed support for PostgreSQL 11, 12, and 13

• Fixed overflow checks in mixed signed/unsigned operations
• Fixed support for 32-bit builds

This release is now ready for general use. The storage format is stable, the input and output functions are robust, and a wide range of functions and operators are implemented.

Also available throughpex.