Adds a (mac_address) field option that switches uint64 fields holding
48-bit MAC addresses to a specialized varint encoder. The fast path
emits exactly 7 bytes when bits [42..47] are non-zero (the common case
for real MACs, since OUIs occupy the top 24 bits) -- one bounds check
and 7 independent stores instead of the 7-iteration shift+branch loop.
calc_uint64_48bit_force mirrors the same fast path in size calculation.
Applied to BluetoothLERawAdvertisement.address, the per-advertisement
address encode in BluetoothLERawAdvertisementsResponse drops from a
serialized per-byte loop to straight-line code.
J. Nick Koston
65f68a412a