This converts the ChunkedReader into an interface and provides two
implementations one sequential and one parallel.
This can be used to improve the performance of the VFS on high
bandwidth or high latency links.
Fixes#4760
Before this change, the VFS layer did not properly handle unicode normalization,
which caused problems particularly for users of macOS. While attempts were made
to handle it with various `-o modules=iconv` combinations, this was an imperfect
solution, as no one combination allowed both NFC and NFD content to
simultaneously be both visible and editable via Finder.
After this change, the VFS supports `--no-unicode-normalization` (default `false`)
via the existing `--vfs-case-insensitive` logic, which is extended to apply to both
case insensitivity and unicode normalization form.
This change also adds an additional flag, `--vfs-block-norm-dupes`, to address a
probably rare but potentially possible scenario where a directory contains
multiple duplicate filenames after applying case and unicode normalization
settings. In such a scenario, this flag (disabled by default) hides the
duplicates. This comes with a performance tradeoff, as rclone will have to scan
the entire directory for duplicates when listing a directory. For this reason,
it is recommended to leave this disabled if not needed. However, macOS users may
wish to consider using it, as otherwise, if a remote directory contains both NFC
and NFD versions of the same filename, an odd situation will occur: both
versions of the file will be visible in the mount, and both will appear to be
editable, however, editing either version will actually result in only the NFD
version getting edited under the hood. `--vfs-block-norm-dupes` prevents this
confusion by detecting this scenario, hiding the duplicates, and logging an
error, similar to how this is handled in `rclone sync`.
