New component requests

[tomcl]

The following components would be useful on Issie:

• Signed and unsigned bus extend
  • Currently there is no way to do this except an ugly one with multiplexor and constants and bus merge!
  • Difficulty - quite easy. Currently there is no exact similar component to clone. It is nbitxor with one input less.
• Not Connected
  • like Viewer but not viewable and does not require a label, is not selectable in waveform simulation.
• N bit merge, N bit split
  • Not easy to make N adjustable because symbol and number of ports must change with N. Currently all ComponentType cases have a fixed number of ports. It is not clear whether this would be a problem.

[tomcl]

Perhaps a better option for Not connected is to make a symbol-level switch to remove or connect to default values/ remove the unused ports.

[tomcl]

Plan:

• Bus extension - can be implemented as library custom components once parameters have been added as in #147
• Merge and split - can be implemented as library custom components if we allow parameters to enable or disable I/O components. That would solve the component size issue automatically.