sudo parses large integer `timestamp_timeout` incorrectly.

[squell]

Describe the bug
If you set timestamp_timeout to a very large value (but within the 18 decimal limit), sudo-rs will panic in debug mode and truncate the timeout to a wrong value in release mode.

To Reproduce
Build sudo + visudo in debug mode, run sudo visudo (ours) and try adding Defaults timestamp_timeout = 157680000000000000. Visudo will fail with a panic.

Expected behavior
Give a parse error.

Context
Caught while proposing a work-around for #1032