Harden union validation

[johnstairs]

The cases in a union type are all required to be distinct. We were failing to catch cases where types were aliased and therefore the union should be invalid, e.g.:

MyInt: int
MyUnion: [int, MyInt]

Secondly, we were missing validation where the type arguments given to a union made up of generic type parameters results in an invalid union, e.g.:

MyGenericUnion<T,U>: [T, U]
MyUnion: MyGenericUnion<int, int>

We now surface an error at the location of the union, with locations where the type arguments are provided.

We were also not catching a union made up of size and unit64 types, which are equivalent types.

Finally, we're changing the representation of the size type in C++ be yardl::Size, an alias of size_t if it is 64-bit, otherwise uint64_t.

Addresses #25