write_bits(011₂)
// Nothing should be written to the file yet.
// In the BitWriter struct object, you will keep track of what would be written.
// ∙ next_byte_to_write will be 011₀₀₀₀₀.
// ∙ 5 bits left
// ∙ 3 bits written

write_bits(1011₂)
// Nothing should be written to the file yet.
// In the BitWriter struct object, you will have the information:
// ∙ next_byte_to_write will be 0111011₀.
// ∙ 1 bits left
// ∙ 7 bits written

write_bits(111₂)
// Write 01110111₂ to the file.
// In the BitWriter struct object, you will have the information:
// ∙ next_byte_to_write will be 11₀₀₀₀₀₀.
// ∙ 6 bits left
// ∙ 2 bits written