* SDK: Add sysvar to expose recent block hashes to programs * Blockhashes is one word * Missed one * Avoid allocs on update * unwrap_or_else * Use iterators * Add microbench * Revert "unwrap_or_else" This reverts commita8f8c3bfbe
. * Revert "Avoid allocs on update" This reverts commit486f01790c
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