See amaranth-lang/rfcs#15 and #784.
Note that this RFC breaks the existing syntax for initializing a view
with a new signal. Instances of `View(layout)` *must* be changed to
`Signal(layout)`.
Found while working on amaranth/rfcs#15. The new code is also robust
in the face of RFC 15 itself (the value returned by Signal will change
but that will save work for the programmer).
