Use bitfields when writing `bits` to avoid reading uninitialized memory

Right now, Emboss uses read-modify-write (RMW) for any values inside `bits`, which means that writing to a field within `bits` can cause undefined behavior (UB) if the underlying buffer was not initialized.

It should be possible, at least on Clang, GCC, and MSVC, to write through a (C++) bitfield without doing the RMW at the C++ level, and therefore avoiding UB.

Note that the exact layout of C++ bitfields is implementation-defined, and there are also potential issues with strict aliasing violations, but for specific compilers it should be doable.