Remove ExprSequence (#4724)

Remove ExprSequence (#4724)

Now that we can break the old API, we don't need an ExprSequence
anymore. This commit removes ExprSequence and replaces all of its uses
with Sequence. Note that I had to temporarily make sequence.h public
because we currently include it in a "public" header because we still
generate the code for Expr::mkConst<Sequence>().