Warn when forgetting to flush stdout before calling std::process::exit.

[ticki]

std::process::exit does not call destructors, thus the buffered standard output should be flushed before calling exit to avoid silencing standard output. A lint ensuring that can potentially create false positives, thus I propose adding it to clippy-pedantic group.

[finnbear]

Maybe this is obsolete? https://internals.rust-lang.org/t/question-what-causes-stdout-to-flush-after-main/12909

[y21]

Closing since nowadays exit() ends up calling io::cleanup() which flushes stdout, so this is no longer needed as a lint