Change SimpleArithmeticEquation to calculate the result and toString value only once during it's creation

[volodymyrkutsenko]

Now SimpleArithmeticEquation getResult() method always calculates the result on its call

Since SimpleArithmeticEquation is an immutable object -- the result cannot change therefore SimpleArithmeticEquation should calculate the result on its creation and getResult() should always return this calculated result.

The same should be done for toString() value