Version 0.8.0

• Adds support for writing a StreamShading with PdfWriter::stream_shading
• Renames Shading to FunctionShading, ShadingType to FunctionShadingType, PdfWriter::shading to PdfWriter::function_shading and ShadingPattern::shading to ShadingPattern::function_shading
• Adds a missing import of the BlendMode type (thanks to @LaurenzV)
• Adds is_empty methods on Array, TypedArray, Dict, and TypedDict