Suggest str methods for trivial regexes

[killercup]

The contains and {starts,ends}_with methods of str can solve many simple regex cases.

cf. #587

[llogiq]

As with the regex validation lint, the trick is using const_eval::eval_const_partial on the regex compile argument, and check for zero or one ^ at the beginning, zero or one $ at the end and characters without special regex meaning in between.

[llogiq]

Also note that the following holds:

Start	End	String method/op
^	$	==
	$	.ends_with(_)
^		.starts_with(_)
		.contains(_)

[llogiq]

We could add suggestions taking the original str (beware of backslashes, though).

[llogiq]

Speaking of backslashes, I found the following false negative:

    let trivial_backslash = Regex::new("a\\.b");
    //~^ERROR: trivial regex
    //~|HELP consider using `str::contains`

[mcarton]

@llogiq I made a PR to fix that false negative. What kind of suggestion do you propose?

[llogiq]

This would require using an ExprUseVisitor to find all usages of the regex and suggest using .contains("a.b") instead.