Sign extend port connections where necessary

Sign extend port connections where necessary

- Signed cell outputs are sign extended when bound to larger wires
- Signed connections are sign extended when bound to larger cell inputs
- Sign extension is performed in hierarchy and flatten phases
- genrtlil indirects signed constants through signed wires
- Other phases producing RTLIL may need to be updated to preserve
  signedness information
- Resolves #1418
- Resolves #2265