0.9.1

This bumps the zstd version to 1.5.1.

It also brings some improvements to the rust wrapper:

• Functions to turn on multithreaded compression are only available when the zstdmt feature is enabled
• Implementations for arrays in zstd-safe use const-generics, and are now gated behind an arrays feature to allow compatibility with old compilers.
• Add set_pledged_src_size to encoders. This can improve the compression ratio, and makes the content size available in the compressed header.
• Add an auto-flush wrapper to the push decoder.