ZstdNet 1.4.5

Main changes: * libzstd v1.4.5 * Span support (#19) * Streaming support (#18) * Advanced parameters APIs support Some other changes: * `libzstd.dll` now cross-compiled using `(i686|x86_64)-w64-mingw32-gcc -DZSTD_MULTITHREAD -DZSTD_LEGACY_SUPPORT=0 -pthread -s` instead of `msvc` for performance reasons * `ZstdException` error codes exposed * On buffers close to 2GB `Wrap` now tries its best, but if source is uncompressible and the size is too large, `ZstdException` with `ZSTD_error_dstSize_tooSmall` will be thrown (#21) * Buffer pooling on `Wrap` without `dst` param Possibly breaking changes: * `Decompressor.GetDecompressedSize()` now throws `ZstdException` instead of returning `0` on error * `ZstdException` with error code `ZSTD_error_dstSize_tooSmall` instead of `InsufficientMemoryException` on `Wrap` if destination buffer too small * `ZstdException` with error code `ZSTD_error_dstSize_tooSmall` instead of `ArgumentOutOfRangeException` on `Unwrap` if destination buffer too small