feat: implement PredicateBoxExt for ?Sized Items

[marienz]

This looks like an oversight: the trait itself already had Item: ?Sized but the generic impl did not.

I noticed this while trying to chain together a variable number of predicate::str::contains(...) with and(). Calling predicates::BoxPredicate::new() explicitly works, but is much wordier than .boxed(), which fails:

173 | pub struct ContainsPredicate {
    | ---------------------------- doesn't satisfy `_: PredicateBoxExt<str>`
    |
    = note: the following trait bounds were not satisfied:
            `str: Sized`
            which is required by `ContainsPredicate: predicates::PredicateBoxExt<str>`
            `&ContainsPredicate: Predicate<_>`
            which is required by `&ContainsPredicate: predicates::PredicateBoxExt<_>`
            `&mut ContainsPredicate: Predicate<_>`
            which is required by `&mut ContainsPredicate: predicates::PredicateBoxExt<_>`

[coveralls]

Pull Request Test Coverage Report for Build 12410765220

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 7.376%

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Totals
Change from base Build 12124300894:	0.7%
Covered Lines:	52
Relevant Lines:	705

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💛 - Coveralls

[marienz]

(I've force-pushed this to fix the capitalization in the commit messages pointed out by the commits lint. Code should not have changed.)