Enable Link-Time Optimization (LTO)

Hi!

I noticed that in the Cargo.toml file Link-Time Optimization (LTO) is not enabled. I suggest turning it on since it will reduce the binary size (always a good thing to have) and highly likely will improve the application's performance.

I suggest enabling LTO only for the Release builds to not sacrifice the developers' experience during the working on the project since LTO consumes an additional amount of time to finish the compilation routine. If you think that a regular Release build should not be affected by such a change as well, then I suggest adding an additional release-lto profile where additionally to regular release optimizations LTO also will be added. Such a change simplifies life for maintainers and others interested in the project persons who want to build the most performant version of ntpd-rs.

Thank you.