unnecessary methods on `CodecPipeline`

[d-v-b]

there are a few methods defined on the CodecPipeline base class that don't need to be methods. Based on their signatures, they are functions that operate on iterables of codecs, so that's probably how we should represent them in our codebase:

• validate
• compute_encoded_size
• evolve_from_array_spec

[clbarnes]

I quite like being able to treat a codecpipeline similarly to an array-to-bytes codec; it was helpful in the n5_default codec impl.

[d-v-b]

treat a codecpipeline similarly to an array-to-bytes codec

this makes sense, and we should define a protocol that formalizes this relationship. but we shouldn't require that people construct a codec pipeline just to validate all the codecs it contains. we should have a free function that validates an iterable of codecs, and the codec pipeline routines should invoke that function as needed.