bencode v0.5.0

Changes:

• More consistent naming by changing event_consumer methods from (begin|end)_(list|dict)
  to (begin|end)_(list|dict).
• Add more benchmarks.
• Disable contract checks in release build for improved performance.
• Fix parsing from a pair of InputItererators.
• Add string_parsing_mode options to push_parser to allow parsing strings to string_view.
• Add experimental SSE4.1 and AVX2 integer parsing backends. Enable SWAR integer parsing by default.
• Add limited std::span serialization/deserialization support.