Add primops to expand the (boxed) array API

GHC is an industry-strength compiler for Haskell. I am proposing to add primitives that allow more efficient operations involving immutable (Small)Array#s, the underlying primitive building blocks for many popular data structures, such as unordered-containers (the go-to hash map library). I will improve efficiency by removing unnecessary initialisation steps that are needed for safety when these operations are implemented in library code.