- Drop `cell_code` and instead map code lookups to the `cell_help` dict.
- Add helper functions to struct for checking and getting the right cell.
- Add `CellType` for cell to `write_cell_rst` function declaration in
preparation for use in future.
- Iterate over `yosys_celltypes.cell_types` when exporting cell rst files,
reporting errors for any cells defined in `cell_types` but not
`cell_help_messages`.
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