Add size, tombstone getters and begin()/end() to iterate over the cells.
Allows inspecting the content of the collection in-place, without
deserializing into an intermediate representation
(collection_mutation_description[_view]). This will be used to gradually
replace all usage of collection_mutation_description[_view].
Deliberately avoding the use of collection_mutation_input_stream as that
one is also a target for elimination.
The names for the accessors are tomb() and size(), following existing
conventions in mutation/. Also rename is_empty() -> empty() to align
with this convention. There is a single caller to update only.
All accessors (new or pre-existing) are made to work with
default-constructed `collection_mutation` (i.e. one containing empty
buffer).
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6a729b4e9e