New functions added:

  * count_ones(): returns number of 1 in a given std_logic_vector
  * xor_reduce(): returns the xor of all bits in a std_logic_vector

Functions even_parity() & odd_parity() now use the xor_reduce function
to calc the parity.

The new common folder holds packages and designs, which can be used
either for synthesis & simulation. The first file is UtilP.vhd, a
package containing vatious helper functions:

  * and_reduce(): function to and'ring all items of a vector, overloaded
                  for std_logic_vector & boolean_vector
  * or_reduce(): function to or'ring all items of a vector, overloaded
                 for std_logic_vector & boolean_vector
  * even_parity(): calculate the even parity of a std_logic_vector
  * odd_parity(): calculate the odd parity of a std_logic_vector