sys_core_fold: Generalize case optimization

For a long time, there has been an optimization for:

  {V1,V2,...} = case Expr of Pat -> ... {Val1,Val2,...}; ... end

that avoids building the tuples. The construct looks like this
in Core Erlang:

  let <V> = case X of
              Pattern -> {Y,Z}
            end
    in case V of
        {A,B} -> A+B
       end

The current optimization will try to replace the second 'case'
with a 'let':

  let <A,B> = case X of
                  Pattern -> <Y,Z>
                end
    in A+B

Simple variations of the construct would prevent the optimizations;
for example this one:

  let <V> = case X of
              Pattern -> {'ok',Val}
            end
    in case V of
        {ok,Val} -> Val
       end

The problem is that the optimization tries to do too much. By
making the optimization do less and have it depend on other
optimizations to finish the job, it will become more powerful.

Thus we can rewrite the code like this:

  let <V1,V2> = case X of
              Pattern -> <'ok',Val>
            end
    in let <V> = {V1,V2}
       in case V of
          {ok,Val} -> Val
       end

Note that the second case is unchanged. The other optimizations in the
sys_core_fold module will optimize the second 'case' and eliminate the
building of the tuple.

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