Deprecate `Record`

[whitequark]

Now that we have RFC 1 and RFC 2, we can finally deprecate (and, later, remove) that awful hack.

It should probably not be removed before Amaranth 0.6, since it is quite widely used.

• Deprecate amaranth.hdl.rec in Amaranth 0.4: hdl.rec: deprecate in favor of lib.data and lib.wiring #890
• Make amaranth.lib.io.Pin an interface object usable with connect: lib.io: make Pin an interface object #891
• Remove amaranth.hdl.rec in Amaranth 0.6