Enable thin LTO for release builds

[lafrenierejm]

See https://doc.rust-lang.org/cargo/reference/profiles.html#release for the release profile's defaults and https://doc.rust-lang.org/cargo/reference/profiles.html#lto for an explanation of link time optimizations (LTO).

Resolves #254, which is where enabling thin LTO was first suggested.

[phiresky]

LTO makes sense definitely, but why do you remove the debug info? It does not affect runtime performance and it's useful because it allows getting backtraces from anyhow and other errors in production builds. It also shouldn't affect binary size since the debug mapping files are kept split (split-debuginfo) so it depends on the packager whether to include them or not.

[lafrenierejm]

…it depends on the packager whether to include [the debug mapping files] or not.

That's fair! I've changed the branch to leave debug info alone and just enable thin LTO.