Tracking issue for RFC 55: New lib.io components

[wanda-phi]

• RFC PR: RFC 55: New lib.io components. rfcs#55
• Implementation PRs:
  • *Port classes: lib.io: Implement *Port from RFC 55. #1214
  • *Buffer classes: lib.io: Implement *Buffer from RFC 55. #1215
  • Using *Buffer in default platform: build.plat: use lib.io.*Buffer in default platform. #1279
  • iCE40 support: vendor._lattice_ice40: implement lib.io buffer primitives. #1281
  • ECP5 support: vendor._lattice_ecp5: implement lib.io buffer primitives. #1295
  • MachXO* support: vendor._lattice_machxo_2_3l: implement lib.io buffer primitives. #1298
  • Xilinx support: vendor._xilinx: implement lib.io buffer primitives. #1303
  • Altera support: vendor._altera: implement lib.io buffer primitives. #1284
  • Gowin support: vendor._gowin: implement lib.io buffer primitives. #1290
  • platform code cleanup: build.{plat,res}: post-lib.io cleanup. #1310
  • deprecations: Implement RFC 55: New lib.io components. #1314
  • documentation: TODO