fix: base_extent calculation in bits_locator_t::template extent_in()

[samvangysegem]

The base_extent variable in the extent_in() method of the bits_locator_t struct now uses the size of its template parameter T instead of std::uint32_t for calculating the base extent .

[elbeno]

I don't think this is right - the Index is always in dwords, hence computing the extent in bytes uses sizeof(std::uint32_t).

[samvangysegem]

You're right... I wrongly assumed the Index of the field_locator_t would correspond to the size of the type being held in the underlying storage of the message. Thanks for clarifying! :)