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

[wanda-phi]

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