Arrays of UInt16

ScalarArray.UInt16Array stores UInt16 values unboxed in a Lean runtime scalar array, 2 bytes per element. Its logical model is Array UInt16: UInt16Array.mk and UInt16Array.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.