Add #[track_caller] to Duration Div impl
Previously the location of the divide-by-zero error condition would be attributed to the code in the rust standard library, eg:
thread 'main' panicked at /home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/time.rs:1172:31:
divide by zero error when dividing duration by scalar
With #[track_caller] the error is correctly attributed to the callee.
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