We would like to be able to forward-declare Telemetry::ID, so fewer
header files need to include Telemetry.h. Empirical evidence shows that
forward-declaring enums requires either enum classes, or plain
fixed-width enums. Let's opt for the latter here; there's no need to
constrain the size to 16 bits, since Telemetry::IDs are rarely stored in
data structures.
