Implement Augmented assignment operations

[simmsb]

Adds an implementation of combined assignment operations

resolves #1

>> let a = 4;
>> a += 3;
>> say a;
7
>> class What { What(x) { this.x = x; } }
>> let b = What(4);
>> say b.x;
4
>> b.x += 6;
>> say b.x;
10
>> b.x /= 2;
>> say b.x;
5

[crazywolf132]

Good work!