serialization/support: corrected SerSequence serialize();¹ tidy.

1: Unused template type parameter `class S` was (rightfully)
causing deduction failure in has_serializer_serialize<Ser, T>.
Also, serialize() cannot take rvalue ref due to no uniform
forwarding, so it now takes lvalue ref.
serializer: removed rvalue capture in has_serializer_serialize.¹

1: It's not possible to get an rvalue ref in serialize() from
the current call paths.
serializer: removed useless dead-path serialization calls.¹

1: Only the failed deductions into serializer::write() and
serializer::read() were being shown. These were not informative
and increased the length of error messages.
serializer: tidy.
serialization/support: remove CV qualifiers for serial configs.¹

1: Sigh. Another perversion to make the matching of serial config
types simpler on serialization functions.
This allows SerX<S, Type, C> to be specified alone to match either C.
Without it, another variation would have to be added with
s/Type/Type const/. In other words, it ensures T is always bare
in the serial config instantiation.
serialization: added SerProxy, make_ser_proxy(), and enum serialize.
test/serialization/general: added SerProxy and enum tests.