This change completes commit 9dc0617e and makes all the tests pass.
It corresponds with the ongoing langauge reference documentation effort.
Fixes#781.
This is actually an existing correctness requirement (for the similar
reasons that ValueCastable.as_value() must always return the same
value every time) that for some reason wasn't respected.
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)`.
* Reject union initialization with more than one reset value.
* Replace the reset value specified in the class definition with
the one provided during initalization instead of merging.
* Annotations like `s: unsigned(4) = 1` are recognized and
the assigned value is used as the reset value for the implicitly
created `Signal`.
* Base classes inheriting from `Struct` and `Union` without
specifying a layout are recognized.
* Classes that both inherit from a base class with a layout and
specify a layout are rejected.
Mutability of Field isn't specified by the RFC and can cause issues
if the objects stored in Layout subclasses are mutated. There isn't
any reason to do that (the subclasses themselves are mutable and
handle that correctly), so disallow it.
