Relax TypeDecorator.process_*_param to return Optional[Any]

[huonw]

The process_bind_param and process_literal_param methods are called to
do decorator-specific conversion of values, before deferring to the
underlying .impl's conversion methods. This means they can return any
value accepted by those methods, not just str (or typing.Text). These
are related to the process_result_value method, which is effectively
the inverse, doing decorator-specific conversion of the output of the
underlying .impl, and indeed this method accepts value: Optional[Any],
representing the unknown type of the output of the underlying .impl.

Unfortunately, modelling this accurately is likely to be
impossible (or at least, much more difficult), because, for instance,
the impl property itself can only be typed as Any, let alone the
input/output type it uses.

Fixes #205

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