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This is a revival of #7490. Quoting #7490: The managed_bytes class now uses implicit linearization: outside LSA, data is never fragmented, and within LSA, data is linearized on-demand, as long as the code is running within with_linearized_managed_bytes() scope. We would like to stop linearizing managed_bytes and keep it fragmented at all times, since linearization can require large contiguous chunks. Large contiguous allocations are hard to satisfy and cause latency spikes. As a first step towards that, we remove all implicitly linearizing accessors and replace them with an explicit linearization accessor, with_linearized(). Some of the linearization happens long before use, by creating a bytes_view of the managed_bytes object and passing it onwards, perhaps storing it for later use. This does not work with with_linearized(), which creates a temporary linearized view, and does not work towards the longer term goal of never linearizing. As a substitute a managed_bytes_view class is introduced that acts as a view for managed_bytes (for interoperability it can also be a view for bytes and is compatible with bytes_view). By the end of the series, all linearizations are temporary, within the scope of a with_linearized() call and can be converted to fragmented consumption of the data at leisure. This has limited practical value directly, as current uses of managed_bytes are limited to keys (which are limited to 64k). However, it enables converting the atomic_cell layer back to managed_bytes (so we can remove IMR) and the CQL layer to managed_bytes/managed_bytes_view, removing contiguous allocations from the coordinator. Closes #7820 * github.com:scylladb/scylla: test: add hashers_test memtable: fix accounting of managed_bytes in partition_snapshot_accounter test: add managed_bytes_test utils: fragment_range: add a fragment iterator for FragmentedView keys: update comments after changes and remove an unused method mutation_test: use the correct preferred_max_contiguous_allocation in measuring_allocator row_cache: more indentation fixes utils: remove unused linearization facilities in `managed_bytes` class misc: fix indentation treewide: remove remaining `with_linearized_managed_bytes` uses memtable, row_cache: remove `with_linearized_managed_bytes` uses utils: managed_bytes: remove linearizing accessors keys, compound: switch from bytes_view to managed_bytes_view sstables: writer: add write_* helpers for managed_bytes_view compound_compat: transition legacy_compound_view from bytes_view to managed_bytes_view types: change equal() to accept managed_bytes_view types: add parallel interfaces for managed_bytes_view types: add to_managed_bytes(const sstring&) serializer_impl: handle managed_bytes without linearizing utils: managed_bytes: add managed_bytes_view::operator[] utils: managed_bytes: introduce managed_bytes_view utils: fragment_range: add serialization helpers for FragmentedMutableView bytes: implement std::hash using appending_hash utils: mutable_view: add substr() utils: fragment_range: add compare_unsigned utils: managed_bytes: make the constructors from bytes and bytes_view explicit utils: managed_bytes: introduce with_linearized() utils: managed_bytes: constrain with_linearized_managed_bytes() utils: managed_bytes: avoid internal uses of managed_bytes::data() utils: managed_bytes: extract do_linearize_pure() thrift: do not depend on implicit conversion of keys to bytes_view clustering_bounds_comparator: do not depend on implicit conversion of keys to bytes_view cql3: expression: linearize get_value_from_mutation() eariler bytes: add to_bytes(bytes) cql3: expression: mark do_get_value() as static