Improve variable store

[JonathanxD]

Currently CodeAPI only increments the variable count instead of reuse variable slots of other frames.

CodeAPI-BytecodeWriter:

public int a(boolean b) {
  if(b) {
    int x = 10; // astore 2
    return x;
  } else {
    int x2 = 11; // astore 3
    return x2;
  }
}

Javac:

public int a(boolean b) {
  if(b) {
    int x = 10; // astore 2
    return x;
  } else {
    int x2 = 11;  // astore 2
    return x2;
  }
}

This issue relates to #4, but issue #4 was dropped, then this issue will cover a particular problem that #4 would solve.