Version 1.20.0

What's Changed

• simplify and harmonize NextSet and NextSetMany by @gaissmai in #191
• explain the slice expression by @gaissmai in #192
• untyped constants, code and comment polishing by @gaissmai in #194
• impl. AppendTo and AsSlice by @gaissmai in #195

Full Changelog: v1.19.0...v1.20.0