Several bugs corrected

[salva]

• The integer comparison function can overflow resulting on bad order
• integers where truncated to 32 bits on amd64
• remove warning caused by int and STRLEN types being different under amd64
• Benchmarks for Perl native sort and Sort::Key were unfair as they were wrapped inside a sub that introduced considerable overhead.
• Use isort instead of ikeysort.
• Remove keysort benchmark as it doesn't make sense.
• Add benchmark for Sort::Key::Radix::isort
• Do not crash when an array with holes is passed
• Sort strings encoded internally as UTF8 correctly
• Add tests for holes and UTF8 bugs