Tracking issue for RFC 65: Special formatting for structures and enums

[wanda-phi]

• RFC PR: Add an RFC for special formatting for structures and enums rfcs#65
• Implementation PRs:
  • add Format.* classes: hdl._ast: add Format.Enum, Format.Struct, Format.Array. #1316
  • use Format.* in lib.enum: lib.enum: add .format() implementation. #1320
  • use Format.* in lib.data: lib.data: add .format() implementation. #1321
  • use Format.* for RTLIL output: hdl._nir, back.rtlil: use Format.* to emit enum attributes and wires for fields. #1323
  • use Format.* for VCD output: sim: use Format.* for VCD output. #1326