utils: fragment_range: add FragmentedView
utils: fragment_range: add FragmentedView

This patch introduces FragmentedView - a concept intented as a general-purpose
interface for fragmented buffers.
Another concept made for this purpose, FragmentedRange, already exists in the
codebase. However, it's unwieldy. The iterator-based design of FragmentRange is
harder to implement and requires more code, but more importantly it makes
FragmentRange immutable.
Usually we want to read the beginning of the buffer and pass the rest of it
elsewhere. This is impossible with FragmentRange.
FragmentedView can do everything FragmentRange can do and more, except for
playing nicely with iterator-based collection methods, but those are useless for
fragmented buffers anyway.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla