Avoid initializing generic vector clones - #572
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Generic.clone allocates its destination with M.new, which initializes the storage before unsafeCopy overwrites every element. Use M.unsafeNew, matching the mutable clone path, to avoid that redundant pass. The copy fills the destination before it is returned.
Fixes #571.
The issue's reproducer reports the cost across Storable and Unboxed vectors and GHC 9.6 through 9.14; it also reports the one-line change passing 2,808 vector-tests-O2 tests and 14 vector-inspection tests on GHC 9.10.3. I confirmed the source paths and copy invariant. This environment has no GHC/Cabal, so I have not run the Haskell tests locally; project CI is needed for validation.