Allow generic argument types

[tonysparks]

Right now generics are extremely primitive in their feature set. I'd like to enhance them to allow to be typed, which would enable two key features:

1. Allowing constant values, which would be useful for declaring constants and array sizes
2. Allowing trait types, which would constrain the generic types allowed.

Example syntax:

public struct StaticArray<SIZE:u32, T> {
   elements: [SIZE]T
}

...
var buffer = StaticArray<16, f32>{}
buffer[0] = 4f

For trait argument types:

trait Hashable<T> {
   hash: func<T>(T>): u32;
}
trait Comparable<T> {
   equals: func<T>(T,T): i32;
}

// Now we know any supplied K must have a method "hash" and V must have a method "equals"
public struct HashMap<K: Hashable<K>, V: Comparable<V>> {
 ...
}