write(011₂)
// We do not write anything to the file, yet.  Instead we will store the state in the
// BitWriter as follows:
//  .current_byte will be 011₀₀₀₀₀
//  .num_bits_left will be 5

write(1010₂)
// We do not write to the file yet.
// BitWriter tracks that:
//  .current_byte will be 0111010₀
//  .num_bits_left will be 1.

write(111₂)
// .current_byte gets completed with the first bit (1).
// .current_byte is (temporarily) 01110101₂.
// We write that byte to the file and reset .current_byte to 00000000₂
// BitWriter now tracks that:
// .current_byte is 11₀₀₀₀₀₀
// .num_bits_left will be 6.