Arrays of UInt8

ScalarArray.UInt8Array stores UInt8 values unboxed in a Lean runtime scalar array, 1 byte per element. Its logical model is Array UInt8: UInt8Array.mk and UInt8Array.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.