Tracking issue for RFC 51: Add `ShapeCastable.from_bits` and `amaranth.lib.data.Const`

[wanda-phi]

• RFC PR: Add an RFC for ShapeCastable.from_bits and lib.data.Const. rfcs#51
• Implementation PR: Implement RFC 51: Add ShapeCastable.from_bits and amaranth.lib.data.Const. #1202
• Deprecation PR (for making ShapeCastable.from_bits non-optional): hdl: make ValueCastable.from_bits override mandatory. #1394

[whitequark]

When ShapeCastable.from_bits is made mandatory, lib.data docs must be updated to remove a few "if from_bits is implemented" clauses.