Implement changesOnly(targetField or getter)

[BenRomberg]

changesOnly prevents modifications on all fields of a class except for those specified.

[BenRomberg]

How to implement:

boolean changesOnly(T fieldOrGetter);

1. analyse stack, require last call being invoke* or getfield
2. for getfield, make sure it's a non-static field of the target class, set all other fields on unchangeable
3. for invoke*, make sure it's a non-static method of the target class returning a value, instrument assuming the last getfield is the one we want