I originally picked :pc: as it is short for "python code", but it is
obscure and :py: is not taken, so a much more obvious role can be used
instead. Also, we all typo :pc: as :py: all the time anyway.
This change completes commit 9dc0617e and makes all the tests pass.
It corresponds with the ongoing langauge reference documentation effort.
Fixes#781.
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).
