In Arduino.h "byte" is typedef'd, but all prototypes except `makeword` use uint8_t, normalize `makeword`

From the compiler's point of view, it's a non-issue, but for the code reader, it's confusing, not least because the inverse functions `lowByte` and `highByte` return type `uint8_t`, not `byte`.
Fixed by #414

Start in Arduino.h and inspect the makeword declaration alongside lowByte and highByte. Confirm the return type is consistent with the other prototypes; the work is done when makeword no longer uses byte and matches the stated uint8_t convention.

Written by the indexing model from the issue text.