Typechecker: Issue type errors for invalid constructor preconditions

The following spec is accepted by the typechecker even though it makes no sense (there is no prestate when the constructor is running):

```
constructor of C
interface constructor()

iff

x == 7

creates

uint x := 2
```

We should modify the typechecker to reject specs containing storage references in constructor precondition blocks.

Start at the typechecker entry point that processes constructor precondition blocks and trace how storage references are validated. Use the provided constructor specification as a regression case; done means the typechecker rejects it because the precondition refers to storage without a prestate.

Written by the indexing model from the issue text.