Renamed some arithmetic functions

1. Renamed functions <current name> (<old name>):

  bit_array_add_uint64  (bit_array_add)
  bit_array_add_word
  bit_array_add_words
  bit_array_sub_uint64  (bit_array_minus)
  bit_array_sub_word    (bit_array_minus_word)
  bit_array_sub_words   (bit_array_minus_words)
  bit_array_mul_uint64  (bit_array_multiply)
  bit_array_div_uint64  (bit_array_div)

  bit_array_add      (bit_array_sum)
  bit_array_subtract (bit_array_difference)
  bit_array_multiply (bit_array_product)
  bit_array_divide

2. Moved decimal functions within bit_array.c to match header file.