Equivalent to C's `signed long long` (`long long`) type.

This type will almost always be [`i64`], but may differ on some systems. The C standard technically only requires that this type be a signed integer that is at least 64 bits and at least the size of a [`long`], although in practice, no system would have a `long long` that is not an `i64`, as most systems do not have a standardised [`i128`] type.

[`long`]: c_long
