Remove restrictions on generic type usage

[sbillig]

What is wrong?

There are some restrictions on the use of generic types that can safely be removed. For example:

fn foo<T>(val: T) {
  val.to_mem() // ERROR: can't call to_mem on generic type
}

We can just allow this, and if T is a primitive type, ignore the function call. Or we could eliminate the to_mem function and this problem would go away.

I think there are others; please add to this issue or create other issues.