add redirection of __Cat__ to allow overrides for more advanced behaviour

add redirection of __Cat__ to allow overrides for more advanced behaviour
without changing fundamental language characteristics or semantics in nmigen

https://bugs.libre-soc.org/show_bug.cgi?id=458

this one is slightly more involved than __Repl__, __Mux__, __Switch__ etc.
because Cat() can receive arbitrary objects including generators.
exactly as is done in what is now _InternalCat, the arguments need to
be flattened and individually Value.cast()ed, however *before* doing
so, the first argument needs to be inspected and treated separately,
because the first argument is the one on which __Cat__ shall be called.

    args[0].__Cat__(*args[1:])

therefore it must not be lowered because its type (a derivative of
UserValue) would be entirely lost through the Value.cast().