What

Minor cleanups. This PR clears the way for this future PR allowing dumping and loading contents to disk.

Specifically:

• Remove unnecessary lib/bitarray-array.rb file
• Move lib/bitarray.rb to lib/bitarray/bit_array.rb, add lib/bitarray.rb that just requires the file from the subdirectory. This makes it easy to add additional classes, later.
• Add reverse_byte accessor
• Make byte_position private
• Move test/test_bitarray.rb to test/test_bit_array.rb

Tests

$ ruby test/test_bit_array.rb 
Run options: --seed 38456

# Running:

.............

Finished in 0.008990s, 1446.1240 runs/s, 468321.7074 assertions/s.

13 runs, 4210 assertions, 0 failures, 0 errors, 0 skips

Additionally, note there are almost no code changes here. Nevertheless, I tested it with my bitarray consisting of a little over 2 billion bits and the code worked identically to that in master.