improve codec encoded size API #4398

we have 2 ways for a codec to report the size it encodes to:

• the is_fixed_size attribute, which defaults to False. Really this should not be an attribute, and it should not default to False.
• the compute_encoded_size method, which only returns int but if it returned None or a sentinel like UNKNOWN then it would completely replace the footgunny is_fixed_size attribute.

you can see where I'm going here. we should soft-deprecate is_fixed_size and widen the return type of compute_encoded_size, allowing that method to express that a particular codec doesn't emit a constant sized output.