Arrays of UInt64

ScalarArray.UInt64Array stores UInt64 values unboxed in a Lean runtime scalar array, 8 bytes per element. Its logical model is Array UInt64: UInt64Array.mk and UInt64Array.toArray convert between the two, and every operation is specified by its effect on the model.

Reads and in-place writes are inline C. uset, set and push update the array in place when it is not shared and copy it otherwise, like Array and ByteArray.