Tracking issue for RFC 55: New `lib.io` components

[wanda-phi]

• RFC PR: amaranth-lang/rfcs#55
• Implementation PRs:
  • [x] *Port classes: #1214
  • [x] *Buffer classes: #1215
  • [x] Using *Buffer in default platform: #1279
  • [x] iCE40 support: #1281
  • [x] ECP5 support: #1295
  • [x] MachXO* support: #1298
  • [x] Xilinx support: #1303
  • [x] Altera support: #1284
  • [x] Gowin support: #1290
  • [x] platform code cleanup: #1310
  • [x] deprecations: #1314
  • [x] documentation: #1364
• Old functionality removal PR (0.6): TODO