Catch instantiation-dependent overlap

```
interface A a
method : a -> Int
instance A Int
method _ = 0
instance A a
method _ = 1

def f (x : a) : Int = method x
```

This is not an error in Dex today, since it can happily commit to the `A a` instance as the only one that satisfies the polymorphic requirements of `method` on the last line. But, if we instantiate `f` at `Int` then we'd probably expect the first instance to trigger! Haskell raises an error in situations like this one and we should too!

Start by tracing instance resolution for the provided Dex example, especially how the polymorphic call to method is committed before f is instantiated at Int. Done means Dex rejects the instantiation-dependent overlap, matching the stated Haskell-like behavior; the payload names no files or tests.

Written by the indexing model from the issue text.