- 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`.
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