Motivation:
There is no reason for `compose()` to be an instance method on `BufferAllocator` since the allocator implementation should not influence how this method is implemented.
Modification:
Make `compose()` a static method and move it to the `Buffer` interface.
Also move its companion methods `extendComposite()` and `isComposite()` to the `Buffer` interface.
Result:
The composite buffer methods are now in a more sensible place.
Also: decided _against_ making `extendComposite()` and `isComposite()` instance methods, because the subtle behaviours of `extendComposite()` means it would behave quite differently for non-composite buffers.
Also: `isComposite()` is not an instance method because it relates to the hard-coded and concrete `CompositeBuffer` implementation.
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