refactor: Optimized clamp_debug_len.

[daniel-wong-dfinity-org]

The problem before is that if the object was huge (e.g. contained a WASM), it would generate the entire String and then, throw away most of it.

One downside of this is that this no longer shows the tail of the object. This is an unfortunate trade off, but it seems to be a net win.

[github-actions]

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[jasonz-dfinity]

The .truncate() works on bytes and it panics if the new_len lies on the char boundary. I don't think it's likely, but we shouldn't panic for debugging.

BTW, for the following suggestion, the simplification to move .push_str("..."); out is just nitpicking. I'm fine with either way.

Suggested change

	if s.len() >= 3 {
	s.truncate(s.len() - 3);
	s.push_str("...");
	} else {
	s.push_str("...");
	}
	if s.len() >= 3 {
	for _ in 0..3 {
	s.pop();
	}
	}
	s.push_str("...");