1286: Add infer for generic default type r=flodiebold a=edwin0cheng
This PR add infer support for generic default type:
```
struct Gen<T=u32> {
val: T
}
```
* add the (unresolved) defaults from the definition to GenericParams
* add a query generic_defaults that resolves those defaults to types and returns a Substs
* add the missing type in `substs_from_path_segment`
* add tests
based on the idea in this [comment](https://github.com/rust-analyzer/rust-analyzer/issues/1099#issuecomment-484206279)
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
3894eb77d8