Add a new method named PKey::EC#to_octet_string that returns the octet
string representation of the curve point. PKey::EC::Point#to_bn, which
have already existed and is similar except that an instance of
OpenSSL::BN is returned, is rewritten in Ruby.

PKey::EC::Point#initialize now takes String as the second argument in
the PKey::EC::Point.new(group, encoded_point) form.

Also, update the tests to use #to_octet_string instead of #to_bn for
better readability.