- Add a type check so that calling Push with an invalid type produces a
  compile error rather than a linker error.

- vi.cpp was calling Push with a variable of type `std::size_t`.
  There's no explicit overload for `size_t`, but there is one for `u64`,
  which on most platforms is the same type as `size_t`.  On macOS,
  however, it isn't: both types are 64 bits, but `size_t` is `unsigned
  long` and `u64` is `unsigned long long`.  Regardless, it makes more
  sense to explicitly use `u64` here instead of `size_t`.