Truncate long block labels

[connorjward]

This makes the tape much much more readable for cases where the __str__ of a block is too large. Since this code path is only touched during visualisation I argue that losing information here is fine.

[JHopeCollins]

This is really useful, thanks! Please can we also allow passing the width through:
Tape.visualise->Tape.visualise_*->Tape.create_graph->Block.create_graph
so it can be set by the user (with the default value you already have)?

Arguably Tape.visualise should call Tape.create_graph directly and pass the graph to each of the visualise_* methods, which would slightly simplify passing through arguments, but that's just style.