Const declarations aren't typechecked

```
const i: str = 10;
fn main() { log i; }
```

Should give an error in typeck, but instead it gets to LLVM, which says

```
Assertion failed: (InitVal->getType() == getType()->getElementType() && "Initializer type must match GlobalVariable type"), function setInitializer, file Globals.cpp, line 168.
```
