3.6.0

Additionally target netstandard2.1 and net8.0.
Added generators and shrinkers for System.Index and System.Range (#621), Memory<T>, ReadOnlyMemory<T>, ReadOnlySequence<T>, ValueTask and ValueTask<T> in netstandard2.1.
Added generators and shrinkers for System.Half, Int128, UInt128, DateOnly, TimeOnly and Rune in net8.0.
Added NullableRef<T> to conveniently generate nulls for reference types (#610).