Make {Try}GetAlternateLookup methods for Dictionary/HashSet instance instead of extensions

[stephentoub]

We made them extension methods instead of instance methods to avoid potential native code size bloat. But the ergonomics of using these without partial generic inference is a bit painful, and we've had reports that it makes them harder to understand. This moves them to be instance methods, and we'll measure the impact on code size to re-evaluate the decision.

[ghost]

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[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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[stephentoub]

Discussed offline with @jkotas, @MichalStrehovsky, @terrajobst, and @eiriktsarpalis. After gathering some data and discussing, it turns out that generic methods (whether or not on a generic type) don't have the same concerning effect on Native AOT code size as do non-generic methods on generic types. We've agreed to go ahead with this improvement.