Merge #757 #758 #759 #760

757: Fix TypePath resolution to iterate segments r=philberty a=philberty

Associated types in rust can be of the form:

```rust
trait Foo {
  type A;
   ...
}

fn test<T:Foo>(a:T) -> T::A { .. }
```

Where the type-bound of Foo is applied to T this allows for a the associated type T::A to exist
which is a placeholder type within the trait definition. This path cannot be resolved at name-resolution
time and requires a path probe.

Fixes #746 

758: The number of required substituions is offset from inherited ones r=philberty a=philberty

When doing HIR::GenericArgs substitutions we must offset from the already
partially substituted arguments.


759: Remove second lookup for query compiled functions r=philberty a=philberty

This cleans up the code here to make use of the query based
compilation which returns the address of the function which
removes extra lookups.


760: Cleanup lookup interfaces r=philberty a=philberty

These patches change the associated impl mapping's to use a boolean
return type for error handling, to stop looking for UNKNOWN_HIRID for
error handling which is too easy to make mistakes with.

It also updates instances of code to use TyTy::ErrorType node instead of
nullptr's for error handling.

Co-authored-by: Philip Herron <philip.herron@embecosm.com>