Improve empty result messages

[alexheretic]

This PR replaces the WARNING: nothing to output message in the cases we can output something more useful.

For example (using the test bench outputs) cargo benchcmp bench_output_4.txt bench_output_5.txt --threshold 2 --improvements yields All (2/2) improvements are within a 2% threshold

Changes

• Add 0/$n benchmarks improved message
• Add 0/$n benchmarks regressed message
• Add All ($n) benchmarks are within a $t% threshold message
• Add All ($r/$n) regressions are within a $t% threshold message
• Add All ($i/$n) improvements are within a $t% threshold message
• Update dependencies

[alexheretic]

@BurntSushi let me know if you can take a look at this, I'll otherwise close this PR in a few days.

[BurntSushi]

@alexheretic Thanks for the ping and the PR! This seems fine to me.