Arrays of USize

ScalarArray.USizeArray stores USize values unboxed in a Lean runtime scalar array, one machine word per element. Its logical model is Array USize: USizeArray.mk and USizeArray.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.