RFR: 7903272: Make method handle methods private

[Athijegannathan Sundararajan]

On Tue, 30 Aug 2022 14:42:45 GMT, Per Minborg <duke at openjdk.org> wrote:

> xxxx$MH methods should be declared private to reduce cluttering in end users' IDEs and elsewhere.
>  
>  This PR also solves esthetic issues in the generated code.

We want to expose native method handles to the user - so that s/he can adapt it rather than just confined to use our wrapper method.

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PR: https://git.openjdk.org/jextract/pull/67