Support for transitive subtype witnesses (#331)

* Change stdlib `saturate` to explicitly specialize `clamp`

This exposes issue #329, and so gives us an easy way to see if transitive subtype witnesses have been implemented correctly.

* Fixup: invoke correct `clamp` overloads

When switching the `clamp` calls in the stdlib definition of `saturate` I made two big mistakes:

1. I was passing in `<T>` in all cases, instead of, e.g., `<vector<T,N>>` in the vector case

2. Of course, the overloads don't actually take `<vector<T,N>>` for the vector case, because `vector<T,N>` is not a `__BuiltinArithmeticType` (`T` is), so instead it should be `clamp<T,N>(...)`.

The issue behind (2) is that we don't support "conditional conformances," which would be a way to say that when `T : __BuiltinArithmeticType` then `vector<T,N> : __BuiltinArithmeticType`. That would be a great long-term wish-list feature, but not something I can see us adding in a hurry.

Anyway the fix here is the simple one: change the vector/matrix call sites to invoke the correct overload in each case.

* Add a notion of transitive subtype witnesses

There are two pieces here:

1. Add the `TransitiveSubtypeWitness` class. This is a witness that `A : C` that works by storing nested subtype witnesses that show that `A : B` and `B : C` for some intermediate type `B`. All the basic `Val` operations are easy enough to define on this.

  - The one gotcha case is whether we can ever simplify away a `TransitiveSubtypeWitness` as part of substitution. That is, if we end up substituting so that both `A` and `B` end up as the same type, then we really just need the `B : C` sub-part. Stuff like that is left as future work.

2. Make the logic in `check.cpp` that constructs subtype witnesses based on found inheritance and constraint declarations able to build up transitive chains. Most of the required infrastructure was already there (the search process maintains a trail of "breadcrumbs" that represent all the steps getting from `A : B` to `B : C` to `C : D` ...).

This change does *not* deal with the required changes in the IR to take advantage of transitive witnesses.

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