Feature to be able to use slice notation on inline parts

[rathinbector]

Users often create designs using sub-sequences of heterologous sequences, which they define as inline parts. In order to make it easier for them to reference and name these sub-sequences in their designs, we should introduce slice notation for inline parts. The syntax of this slice notation should work exactly the same as the current syntax for slice notation on regular parts.

@daz10000, @legezam