gh-125420: implement Sequence.__contains__ API on memoryview objects

[picnixz]

• Issue: memoryview is a Sequence but does not implement the full Sequence API #125420

[picnixz]

Converting into a draft to decide whether the pure iterator approach is actually efficient enough or if iteration using the underlying multi-dimensional structure would be preferred (without spawning an iterator).

[picnixz]

For now, let's just keep this implementation. It's probably faster than the generic implementation since there are less code paths but it could definitely be faster by iterating directly over the buffer (and we don't seem to support iterating over multi-dimensional buffers yet).