[C++] Unify BasicDecimal128 and BasicDecimal256 classes

[asfimport]

BasicDecimal128 uses two dedicated int64 (higher, lower) to store the value. It's possible to use std::array<uint64, 2> to store the value, just like BasicDecimal256 does, and to unify the two classes.
As decimal operations are quite expensive, guess this indirection won't cause performance loss.

Related issues:

• [C++] Make Decimal representations consistent (is related to)

_{Note: This issue was originally created as ARROW-13439. Please see the migration documentation for further details.}

[asfimport]

yibocai#1:
Looks it doesn't deserve the effort.
One possible approach is to derive BasicDecimal128 and 256 from a templated base class.
It does reduces many common code. But the problem is I have to put all implementation code in header file, which is too big.

template <int BitWidth>
class BasicDecimal {
  ......
  std::array<uint64_t, BidWidth/64> array_;
}

class BasicDecimal128 : public BasicDecimal<128> {
  ......
};

class BasicDecimal256 : public BasicDecimal<256> {
  ......
};