Issue a warning when assigning something with a mutable field to something with a non-mutable field

The following code does compile, and should compile:

```
o1 : {foo: var int} = {foo: var 0}

o2 : {foo: int} = o1

o1.foo = 1

o2.foo // 1
```

However, it's still dangerous. The type checker should issue a warning in this type of scenario.

Similar for arrays or other sequence types.