Find better way to handle special WireVector subclasses like `_MemIndexed`

[mdko]

Special WireVector classes (beyond the normal Register/Const/Input/Output) like _MemIndexed, while not user-facing, end up being exposed nonetheless when used in conjunction with block methods. For example, see #405, which added a special error message to help alert the user why using the result of a memory read doesn't work as a key into the set of source/destination nets the block returns.

I suspect similar issues may occur when using a _MemIndexed as the input to a net transform, etc., so this issue is a call for further investigation and testing and possible internal rewrites to how _MemIndexed is implemented.