Provide extra analysis for helping with weight files inspection

[joao-paulo-parity]

The following report is from @athei

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Tell us your story

Whenever I run the bench bot on the pallet_contracts I need to go through the diff to see whether some benchmark result has substantially changed. And there are a lot of benchmarks.

It would be nice if the benchbot could analyze the diff for me and post a summary that contains:

• Benchmarks that did substantially change their weight (like 50% or something)
• New benchmarks
• Removed benchmarks

What tools have you already tried

None. The problem isn't annoying me enough to do something about it.

How bad is the pain?

• Itching from time to time. I'm open for discussion
• Fairly intense. I'd really like to have a solution soon
• LITERALLY BLOCKER

Additional info

I realize that this is mostly a problem for contracts and that this might be of low priority. Still, I wanted to write it down somewhere.