Docs: Improve cell_help usage

- 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`.

techlibs/common/cellhelp.py

@@ -22,18 +22,14 @@ class SimHelper:
             "name", "title", "ports", "source", "desc", "code", "ver",
         ]
         # generate C++ struct
-        val = "tempCell = {\n"
+        val = f"cell_help[{json.dumps(self.name)}] = "
+        val += "{\n"
         for field in printed_fields:
             field_val = getattr(self, field)
             if isinstance(field_val, list):
                 field_val = "\n".join(field_val)
             val += f'  {json.dumps(field_val)},\n'
         val += "};\n"
-
-        # map name to struct
-        val += f'cell_help[{json.dumps(self.name)}] = tempCell;'
-        val += "\n"
-        val += f'cell_code[{json.dumps(self.name + "+")}] = tempCell;'
         return val
 
 def simcells_reparse(cell: SimHelper):