Variable merging

```
declare var test: (s: string) => void;

declare var test: {
(s: string) => void;
tests: any[];
}
```

At the moment TS compiler shows error: `Subsequent variable declarations must have the same type.`

Start by reproducing the declaration example in the TypeScript compiler and tracing the diagnostic for subsequent variable declarations. The work is done when the compatible declarations merge as requested while incompatible subsequent declarations continue to produce an error.

Written by the indexing model from the issue text.